arm64: add additional MDCR_EL2 fields

Monitor Debug Configuration Register provides EL2 configuration options
for self-hosted debug and the Performance Monitors Extension.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46191

(cherry picked from commit 610348a90467980de0498fab8dfdddf221d7a604)