arm64: Add register definitions for MDCR_EL2

This is needed to support the bhyve gdb stub implementation on arm64.

Reviewed by: andrew
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D42867

(cherry picked from commit 4f12883c360dbb03562f11713e711e3a3a330a0f)