mips32, implement assembler li instruction

Implement it as a function, the code was already in. Added optimize
option.

Change-Id: Ib9ad3f00d6c4f0b91c4e4960a50ec8d102f4e333
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/4017


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