arm64: mte: add system register definitions

Add system register and bit field definitions for Memory Tagging
Extension (MTE) in ARMv8.5.

Reviewed by: andrew
Sponsored by: Arm Ltd
Signed-off-by: Harry Moulton <harry.moulton@arm.com>
Co-authored-by: Andrew Turner <andrew@FreeBSD.org>
Differential Revision: https://reviews.freebsd.org/D55945