mips: use cp0 DeSave to cache $15 / t7

mips: use cp0 DeSave to cache $15 / t7

Near all pracc functions store $15 in DeSave and
restore it when exiting.
There is no need to save it, if mips32_pracc_read_regs()
save this register in Desave when entering debug mode.
mips32_pracc_write_regs() needs to update it when
exiting debug mode.
Other pracc functions must not modify DeSave.
The jump code in the fastdata transfer function needs also
some little modifications.
Remark:
Like in current code the user can read/modify $15
with the cp0 31 commands.

Change-Id: I5b7dfc1b6169da846f5d2dd3ad4209a9da2c3fad
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/1565
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>