Add support for arm64 to the existing arm generic timer driver:
- Add macros to handle the differences in accessing these registers on arm and arm64.
- Use the fdt data to detect if we are on an ARMv7 or ARMv8.
- Use the virtual timer by default on arm64, we may not have access to the physical timer.
Differential Revision:	https://reviews.freebsd.org/D2208
Reviewed by:	emaste
Sponsored by:	The FreeBSD Foundation