arm64: add HCRX_EL2 register

HCRX_EL2 is the Extended Hypervisor Configuration Register introduced
with FEAT_HCX in ARMv8.7. All fields in this register are used for
features in ARMv8.7 and above. Initially zero the register, incase
firmware has not properly configured it.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D48583
Signed-off-by: Harry Moulton <harry.moulton@arm.com>