KVM: arm64: Add predicate for FPMR support in a VM

As we are about to check for the advertisement of FPMR support to
a guest in a number of places, add a predicate that will gate most
of the support code for FPMR.

Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240820131802.3547589-3-maz@kernel.org


Signed-off-by: Marc Zyngier <maz@kernel.org>