timer-uclass: Always use "clock-frequency" property as fallback

Currently the "clock-frequency" DT property is only being considered as an
fallback if either there is no clock driver, the clock driver implements
the request-op correctly or there is no clock defined for the timer at all.

This patch makes "clock-frequency" also being picked as a fallback if
getting the clock-rate fails, since clk_get(_by_index) will return no
error, if a clock driver does not implement the request-op and does also
not support getting the rate of the clock in question.
timer_post_probe will take care if the property does not exist in the DT or
is defined as 0.

Signed-off-by: Alex Bee <knaerzche@gmail.com>