mips32: add micromips isa handling

Read and save configuration registers, up to 4.
Config3 holds the micromips implementation info.
Added isa implementation info to mips32_common.
Added isa filter to avoid common mistakes, but only
if one isa mode is implemented.
When resuming the isa requested is set if more than
one isa mode is implemented.

Change-Id: I1d6526c5525bffac8d75e031b842b2edc6310e28
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/4123


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>