arm64: Add EL1 and EL12 register alt names

When adding VHE support we may need to access these registers using
the _EL12 or _EL1 name depending on if VHE is enabled or not.

Add the registers we will likely need to access from VHE so these can
be updated in the guest switcher code.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45525