arm64/vmm: Clean up enabling guest timer access

Clean up the name of CNTHCTL_EL2 field macros and expand to include
more fields. This makes it easier to see which accesses are trapped or
not trapped.

While here set the register directly. We already set it in locore.S so
there is no need to read that and modify it.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D51817