ARM: imx: clk-vf610: get input clocks from assigned clocks

With the clock assignment device tree changes, the clocks get
initialized properly but the search for those clocks fails with
errors:

[    0.000000] i.MX clk 4: register failed with -17
[    0.000000] i.MX clk 5: register failed with -17

This is because the module can't find those clocks anymore, and
tries to initialize fixed clocks with the same name.

Get the clock modules input clocks from the assigned clocks by
default by using of_clk_get_by_name(). If this function returns
not a valid clock, fall back to the old behaviour and search the
input clock from the device tree's /clocks/$name node.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>