RISC-V: Split out the XIP fixups into their own file

This was broken by the original refactoring (as the XIP definitions
depend on <asm/pgtable.h>) and then more broken by the merge (as I
accidentally took the old version).  This fixes both breakages, while
also pulling this out of <asm/asm.h> to avoid polluting most assembly
files with the XIP fixups.

Fixes: bee7fbc3 ("RISC-V CPU Idle Support")
Fixes: 63b13e64 ("RISC-V: Add arch functions for non-retentive suspend entry/exit")
Link: https://lore.kernel.org/r/20220420184056.7886-4-palmer@rivosinc.com


Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>