HCRX_EL2.EnFPM controls the trapping of FPMR (as well as the validity
of any FP8 instruction, but we don't really care about this last part).

Describe the trap bit so that the exception can be reinjected in a
NV guest.
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240820131802.3547589-6-maz@kernel.orgSigned-off-by: Marc Zyngier <maz@kernel.org>