arm64/vmm: Add a feature flag and use it for HCRX

[andrew]

Summary

Add a field to hold the features the hardware supports that need to be
handled when switching to a guest and use it to handle FEAT_HCX that
adds the HRX_EL2 register.

This reduces the number of times we read ID registers in guest
switching which may be trapped when running under nested virtualisation.

Sponsored by: Arm Ltd