arm64: Don't use hex for ID_AA64MMFR2_EL1_op/CR*

It breaks a future macro that creates the alternative register name
for old compilers.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D40892

(cherry picked from commit 2134cfe793d67e3bc6ec0713862168452f4ad8f0)