arm: Move contents of cpu-v6.h into cpu.h

Previously we had an armv4/v5 and armv6/v7 copy of the cpu.h header. As
we have removed armv4/v5 support we can now merge the armv6/v7 code
into cpu.h

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D41137