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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+N1213 is a configurable hard/soft core of NDS32's N12 CPU family.
+
+Features
+========
+
+CPU Core
+ - 16-/32-bit mixable instruction format.
+ - 32 general-purpose 32-bit registers.
+ - 8-stage pipeline.
+ - Dynamic branch prediction.
+ - 32/64/128/256 BTB.
+ - Return address stack (RAS).
+ - Vector interrupts for internal/external.
+   interrupt controller with 6 hardware interrupt signals.
+ - 3 HW-level nested interruptions.
+ - User and super-user mode support.
+ - Memory-mapped I/O.
+ - Address space up to 4GB.
+
+Memory Management Unit
+ - TLB
+   - 4/8-entry fully associative iTLB/dTLB.
+   - 32/64/128-entry 4-way set-associati.ve main TLB.
+   - TLB locking support
+ - Optional hardware page table walker.
+ - Two groups of page size support.
+  - 4KB & 1MB.
+  - 8KB & 1MB.
+
+Memory Subsystem
+ - I & D cache.
+   - Virtually indexed and physically tagged.
+   - Cache size: 8KB/16KB/32KB/64KB.
+   - Cache line size: 16B/32B.
+   - Set associativity: 2-way, 4-way or direct-mapped.
+   - Cache locking support.
+ - I & D local memory (LM).
+   - Size: 4KB to 1MB.
+   - Bank numbers: 1 or 2.
+   - Optional 1D/2D DMA engine.
+   - Internal or external to CPU core.
+
+Bus Interface
+ - Synchronous/Asynchronous AHB bus: 0, 1 or 2 ports.
+ - Synchronous High speed memory port.
+   (HSMP): 0, 1 or 2 ports.
+
+Debug
+ - JTAG debug interface.
+ - Embedded debug module (EDM).
+ - Optional embedded program tracer interface.
+
+Miscellaneous
+ - Programmable data endian control.
+ - Performance monitoring mechanism.