clk: clk_set_default: accept no-op skip fields

The Assigned Clock parents and rates misses the fact that a "0" entry can
be passed to skip setting a parent or rate of an assigned clock as
described in the Linux clock bindings at [1].

This patch simply skips the clock reparenting if the DT parsing returns
-ENOENT and the clock rate setting if "0" is passed as clock rate.

[1] https://github.com/torvalds/linux/blob/master/Documentation/devicetree/bindings/clock/clock-bindings.txt#L135

Fixes: f4fcba5c5baa "clk: implement clk_set_defaults()"
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>