arm64: Enabling new hypercalls using HvCallSetVpRegisters and HvCallGetVpRegisters

Enabling HvCallSetVpRegisters and HvCallGetVpRegisters for hypercalls to
read and write to specific MSRs. This is required for implementing wrmsr
and rdmsr, which is required for Hyper-V vmbus driver for ARM64.

Also we need to use arm smccc hvc 1.2 version as we need to access
registers beyond X0-X3 for HvCallGetVpRegisters. Currently scoping
it only for Hyper-V.

Reviewed by: lwhsu, andrew, whu
Tested by: Souradeep Chakrabarti <schakrabarti@microsoft.com>
Signed-off-by: Souradeep Chakrabarti <schakrabarti@microsoft.com>
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D36256