UPSTREAM: arm64: insn: Add aarch64_insn_decode_immediate

Patching an instruction sometimes requires extracting the immediate
field from this instruction. To facilitate this, and avoid
potential duplication of code, add aarch64_insn_decode_immediate
as the reciprocal to aarch64_insn_encode_immediate.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Bug: 31432001
Change-Id: If95995457fb8341f5ec0e74b0e0db2f737d68822
(cherry picked from commit 0978fb25f86b7595821cee6955679250d47c6438)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>