timer: Add helper for drivers using timebase fallback

timer: Add helper for drivers using timebase fallback

This function is designed to be used when a timer used to be initialized by
the cpu (e.g. RISC-V timers), but now is initialized by dm_timer_init. In
such a case, the timer may prefer to use the clocks and clock-frequency
properties, but should be able to fall back on using the cpu's
timebase-frequency.

Signed-off-by: Sean Anderson <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Reviewed-by: Rick Chen <[email protected]>