mips: optimize mips32_pracc_read_regs() code

mips: optimize mips32_pracc_read_regs() code

Current code needs 101 pracc accesses for this function, this code needs 12 less.
There is a singularity in this code, is the only function that restore
a register from param out instead from  pracc stack. Obviously the register
was previously stored at param out. This save 2 pracc accesses.

Change-Id: Ie95b6f983a3198dafc0eab2dd5acc11f871a8d83
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/958
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>