altos: Timers clock base depends on perhipheral bus prescalers too

altos: Timers clock base depends on perhipheral bus prescalers too

For some weird reason, a non-unity perhipheral bus clock scaler
affects the base of the various timers; this left the 100Hz tick
running at 200Hz.

Signed-off-by: Keith Packard <keithp@keithp.com>