clk: imx6q: add ethernet refclock mux support

Add ethernet refclock mux support and set it to internal clock by
default. This configuration will not affect existing boards since
machine code currently overwrites this default.

The machine code will be fixed in a separate patch.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Link: https://lore.kernel.org/r/20230131084642.709385-3-o.rempel@pengutronix.de