KVM: arm64: Streamline save/restore of HFG[RW]TR_EL2

The way we save/restore HFG[RW]TR_EL2 can now be simplified, and
the Ampere erratum hack is the only thing that still stands out.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240214131827.2856277-21-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>