mips: optimize read code for speed

mips: optimize read code for speed

Really nothing new that not explained in previous patches.
The code is expanded as needed, there are no loops in pracc code.
For the first value pracc accesses are reduced from 39 to 16
and for aditional values from 10 to 3.
dump_image should work around 3x faster.

Change-Id: I37c9b13395c09eb52a91f10cdb6cbaedef8ab98b
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/955
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>