MIPS: Add uasm_i_dsrl_safe() and uasm_i_dsll_safe() to uasm.

This allows us to clean up the code by not having to explicitly code
checks for shift amounts greater than 32.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/1153/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>