arm64: insn: Add barrier encodings

Create necessary functions to encode/decode aarch64 barrier
instructions.

DSB needs special case handling as it has multiple encodings.

Signed-off-by: Julien Thierry <jthierry@redhat.com>
Link: https://lore.kernel.org/r/20210303170536.1838032-7-jthierry@redhat.com


[will: Don't reject DSB #4]
Signed-off-by: Will Deacon <will@kernel.org>