MIPS: mm: uasm: Add signed 9-bit immediate related macros

MIPS R6 redefines several instructions and reduces the immediate
field to 9-bits so add related macros for the microassembler.

Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>