Add support for arm64 to the arm generic timer driver

[andrew]

Summary

• 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.

[emaste]

sys/arm/arm/generic_timer.c

109

Perhaps #else /* __aarch64__ */ just to make it clear where else this is used.

[andrew]

Add #else /* __aarch64__ */

[emaste]

looks good to me

[andrew]

Closed by commit rS281072 (authored by @andrew).