On some configuration we may get following error:
[ 0.000000] imx:clk-gpr-mux: failed to get parent (-EINVAL)
This happens if selector is configured to not supported value. To avoid
this warnings add dummy parents for not supported values.
Fixes: 4e197ee880c2 ("clk: imx6ul: add ethernet refclock mux support")
Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>