KVM: arm64: nv: Advertise the lack of AArch32 EL0 support

Although we never supported 32bit anywhere in NV, we fail to
advertise so for EL0, probably owing to the relative lack of
hardware supporting both NV2 and 32bit EL0.

Add some sanitising to ID_AA64PFR0_EL1.EL0, and reaffirm that
"in 64bit-only we trust".

Reported-by: Oliver Upton <oliver.upton@linux.dev>
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>