[MIPS] lib_mips/time.c: Fix CP0 count register usage and timer routines

MIPS port has two problems in timer routines. One is now we assume CFG_HZ
equals to CP0 counter frequency, but this is wrong. CFG_HZ has to be 1000
in the U-Boot system.

The other is we don't have a proper time management counter like timestamp
other ARCHs have. We need the 32-bit millisecond clock counter.

This patch introduces timestamp and CYCLES_PER_JIFFY. timestamp is a
32-bit non-overflowing CFG_HZ counter, and CYCLES_PER_JIFFY is the number
of calculated CP0 counter cycles in a CFG_HZ.

STRATEGY:

* Fix improper CFG_HZ value to have 1000

* Use CFG_MIPS_TIMER_FREQ for timer counter frequency, instead.

* timer_init: initialize timestamp and set up the first timer expiration.
  Note that we don't need to initialize CP0 count/compare registers here
  as they have been already zeroed out on the system reset. Leave them as
  they are.

* get_timer: calculate how many timestamps have been passed, then return
  base-relative timestamp. Make sure we can easily count missed timestamps
  regardless of CP0 count/compare value.

* get_ticks: return the current timestamp, that is get_timer(0).

Most parts are from good old Linux v2.6.16 kernel.

v2:
- Remove FIXME comments as they turned out to be trivial.
- Use CP0 compare register as a global variable for expirelo.
- Kill a global variable 'cycles_per_jiffy'. Use #define CYCLES_PER_JIFFY
  instead.

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>

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