arm64: insn: Add load/store decoding helpers

Provide some function to group different load/store instructions.

Signed-off-by: Julien Thierry <jthierry@redhat.com>
Link: https://lore.kernel.org/r/20210303170536.1838032-9-jthierry@redhat.com


Signed-off-by: Will Deacon <will@kernel.org>