arm64/sme: Manually encode ZT0 load and store instructions

In order to avoid unrealistic toolchain requirements we manually encode the
instructions for loading and storing ZT0.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-sme2-v4-6-f2fa0aef982f@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>