doc/README: documents and readme for NDS32 arch

Documents and READMEs for NDS32 architecture.
It patch also provides usage of SoC AG101 and board ADP-AG101.

Signed-off-by: Macpaul Lin <macpaul@andestech.com>

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+Andes Technology SoC AG101
+==========================
+
+AG101 is the first SoC produced by Andes Technology using N1213 CPU core.
+AG101 has integrated both AHB and APB bus and many periphals for application
+and product development.
+
+ADP-AG101
+=========
+
+ADP-AG101 is the SoC with AG101 hardcore CPU.
+
+Please check http://www.andestech.com/p2-4.htm for detail of this SoC.
+
+Configurations
+==============
+
+CONFIG_MEM_REMAP:
+	Doing memory remap is essential for preparing some non-OS or RTOS
+	applications.
+
+	This is also a must on ADP-AG101 board.
+	(While other boards may not have this problem).
+
+	The reason is because the ROM/FLASH circuit on PCB board.
+	AG101-A0 board has 2 jumpers MA17 and SW5 to configure which
+	ROM/FLASH is used to boot.
+
+	When SW5 = "0101", MA17 = LO, the ROM is connected to BANK0,
+	and the FLASH is connected to BANK1.
+	When SW5 = "1010", MA17 = HI, the ROM is disabled (still at BANK0),
+	and the FLASH is connected to BANK0.
+	It will occur problem when doing flash probing if the flash is at
+	BANK0 (0x00000000) while memory remapping was skipped.
+
+	Other board like ADP-AG101P may not enable this since there is only
+	a FLASH connected to bank0.
+
+CONFIG_SKIP_LOWLEVEL_INIT:
+	If you want to boot this system from FLASH and bypass e-bios (the
+	other boot loader on ROM). You should undefine CONFIG_SKIP_LOWLEVEL_INIT
+	in "include/configs/adp-ag101.h".
+
+Build and boot steps
+====================
+
+build:
+1. Prepare the toolchains and make sure the $PATH to toolchains is correct.
+2. Use `make adp-ag101` in u-boot root to build the image.
+
+burn u-boot to flash:
+1. Make sure the MA17 (J16) is Lo.
+2. Make sure the dip switch SW5 is set to "0101".
+3. Power On. Press button "S1", then press button "SW1", then you will found the
+   debug LED show 67 means the system successfully booted into e-bios.
+   Now you can control the e-bios boot loader from your console.
+4. Under "Command>>" prompt, enter "97" (CopyImageFromCard)
+5. Under "Type Dir Name of [CF/SD] =>" promtp, enter "c".
+6. Under "Enter Filename =>" prompt, enter the file name of u-boot image you
+   just build. It is usually "u-boot.bin".
+7. Under "Enter Dest. Address =>" prompt, enter the memory address where you
+   want to put the binary from SD card to RAM.
+   Address "0x500000" is our suggestion.
+8. Under "Command>>" prompt again, enter "55" (CLI) to use interactive command
+   environment.
+9. Under "CLI>" prompt, enter "burn 0x500000 0x80400000 0x30000" to burn the
+   binary from RAM to FLASH.
+10. Under "CLI>" prompt, enter "exit" to finish the burn process.
+
+boot u-boot from flash:
+1. Make sure the MA17 (J16) is Hi).
+2. Make sure the dip switch SW5 is set to "1010".
+3. Power On. Press button "S1", then you will see the debug LED count to 20.
+4. Now you can use u-boot on ADP-AG101 board.