arm64: insn: Add some opcodes to instruction decoder

Add decoding capability for some instructions that objtool will need
to decode.

Signed-off-by: Julien Thierry <jthierry@redhat.com>
Link: https://lore.kernel.org/r/20210303170536.1838032-8-jthierry@redhat.com


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