timer: Support clocks via phandle

timer: Support clocks via phandle

Earlier timer driver needed a clock-frequency property in compatible
device-tree nodes. Another way is to reference a clock via a phandle.

So now timer_pre_probe tries to get clock by reference through device
tree. In case it is impossible to get clock device through the
reference, clock-frequency property of the timer node is read to provide
backward compatibility.

Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Reviewed-by: Simon Glass <sjg@chromium.org>