Xilinx: ARM: timer: Correct error handling

With this patch timer init falls back
to device tree information if the timer clock is not found through
the clock framework. The DT path in turn falls back to hard
coded defaults if no matching node and property is found.
The behavior in case the clock lookup fails is the same as the
pre-common_clk behavior.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>