clk: imx6ul: fix "failed to get parent" error

clk: imx6ul: fix "failed to get parent" error

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@pengutronix.de>
Link: https://lore.kernel.org/r/20230310164523.534571-1-o.rempel@pengutronix.de
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>