mips: optimize mips32_pracc_read_u32() function

mips: optimize mips32_pracc_read_u32() function

This function is highly optimized, there is not much to
improve.
Loading the base address for pracc access with the new
defined MIPS32_PRACC_BASE_ADDR saves one instruction.
The memory address is loaded in too steps. First the upper
address is loaded. The lower address is passed as an offset in
the memory load instruction.
The offset is signed, if the lower address is in the range of
0x8000 to 0xffff the offset is a negative value, and the upper
address must be incremented by 1.
Pracc accesses are now 12 instead of 14.

Change-Id: I286945b240ed5c5d5cc540780a41a8a5fa075da3
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/952
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>