clk: imx6ul: 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.

clock tree before this patch:
fec1 <- enet1_ref_125m (gate) <- enet1_ref (divider) <-,
                                                       |- pll6_enet
fec2 <- enet2_ref_125m (gate) <- enet2_ref (divider) <-´

after this patch:
fec1 <- enet1_ref_sel(mux) <- enet1_ref_125m (gate) <- ...
               `--<> enet1_ref_pad                      |- pll6_enet
fec2 <- enet2_ref_sel(mux) <- enet2_ref_125m (gate) <- ...
               `--<> enet2_ref_pad

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Lee Jones <lee@kernel.org>
Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Link: https://lore.kernel.org/r/20230131084642.709385-17-o.rempel@pengutronix.de