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