MIPS: Add uasm UASM_i_SRL_SAFE macro.

This can be used from either 32-bit or 64-bit code to generate logical
right shifts of any constant amount.

Signed-off-by: David Daney <david.daney@cavium.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2576/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>