imx8mn-var-som: Simplify FEC initialization

imx8mn-var-som: Simplify FEC initialization

With DM enabled, there is no need for board code to initialize
the FEC interface.

The ethernet PHYs on the symphony carrier board have a dedicated
crystal/oscillator. If the SOM has the EC configuration option
(onboard ethernet PHY), it also has a dedicated crystal/oscillator.
So in all cases, there is no need to enable the RGMII TX clk output.

This follows a similar change introduced by Fabio Estevam:
Link: https://lore.kernel.org/u-boot/20231019170441.1610453-1-festevam@gmail.com/
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>