MIPS: Move definitions for 32/64-bit agonstic inline assembler to new file.

Inspired by Markos Chandras' patch.  I just didn't want do pull bitsops.h
into pgtable.h.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
References: https://patchwork.linux-mips.org/patch/11052/