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

(cherry picked from commit f38792ffc2db67ca82026eb4861f37fe7ac7c38e)