Add machine/sysreg.h to simplify accessing the system control coprocessor
registers and use it in the ARMv7 CPU functions.

The sysreg.h file has been checked by hand, however it may contain errors
with the comments on when a register was first introduced. The ARMv7 cpu
functions have been checked by compiling both the previous and this version
and comparing the md5 of the object files.

Submitted by: Svatopluk Kraus <onwahe at gmail.com>
Submitted by: Michal Meloun <meloun at miracle.cz>
Reviewed by: ian, rpaulo
Differential Revision: https://reviews.freebsd.org/D795