a10_timer: some tidying/re-organization

In preparation for enabling the driver on the D1 platform (riscv).

• Better isolate armv7-only portions of the driver
• Move a10 timecounter setup into a helper function
• Small style tweaks

Reviewed by: manu, ganbold
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48670