[ARM] Orion: prepare for runtime-determined timer tick rate

Currently, orion5x uses a hardcoded timer tick rate of 166 MHz, but
the actual timer tick rate varies between different members of the SoC
family (and can vary based on strap pin settings).

This patch prepares for runtime determination of the timer tick rate.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>