Update the SCTLR_EL1 register definitions

They are valid as of the ARMv8.7 XML.

While here remove SCTLR_RES0 as it's unused and depends on which CPU
the kernel is running on and switch to shifted values as they are
easier to compare with the documentation.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31120