arm64/fpsimd: Enable host kernel access to FPMR

arm64/fpsimd: Enable host kernel access to FPMR

FEAT_FPMR provides a new generally accessible architectural register FPMR.
This is only accessible to EL0 and EL1 when HCRX_EL2.EnFPM is set to 1,
do this when the host is running. The guest part will be done along with
context switching the new register and exposing it via guest management.

Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240306-arm64-2023-dpisa-v5-2-c568edc8ed7f@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>