mips: optimize write code for speed

mips: optimize write code for speed

All the writes are done by the new function mips32_pracc_write_mem_generic().
The code is similar to the read generic code.
The reuse of register 15 as memory base address saves 3 pracc accesses.
The first write takes 13(12) pracc accesses and for additional writes 3(2).
Loading miniprograms should take 25% less time and loading fastdata transfer
handler code should be over 2x faster.

Change-Id: Ia3b24ba084af33be99da19f00a7fd4d1b291f350
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/956
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>