mips: optimize CP0 read/write code

mips: optimize CP0 read/write code

MIPS32_PRACC_BASE_ADDR is defined as 0xFF200000. Now is
possible to load the base address with a lui instruction and
only one pracc access.
Offsets to the pracc code addresses are defined to simplify the code
and probably make it a bit more readable or self-explained.

Change-Id: I853dd2d7fad52745931cc6e6be68c0ae156d897e
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/951
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Tested-by: jenkins