arm64: Document boot requirements for Guarded Control Stacks

FEAT_GCS introduces a number of new system registers, we require that
access to these registers is not trapped when we identify that the feature
is present.  There is also a HCRX_EL2 control to make GCS operations
functional.

Since if GCS is enabled any function call instruction will cause a fault
we also require that the feature be specifically disabled, existing
kernels implicitly have this requirement and especially given that the
MMU must be disabled it is difficult to see a situation where leaving
GCS enabled would be reasonable.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-6-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

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