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>

board/variscite/imx8mn_var_som/imx8mn_var_som.c

@@ -12,7 +12,6 @@
 #include <fdt_support.h>
 #include <i2c_eeprom.h>
 #include <malloc.h>
-#include <asm/io.h>
 #include <asm/global_data.h>
 #include <dt-bindings/gpio/gpio.h>
 #include <linux/libfdt.h>
@@ -46,20 +45,8 @@ struct var_imx8_eeprom_info {
 	u8 partnumber2[5];        /* Part number 2 */
 } __packed;
 
-static void setup_fec(void)
-{
-	struct iomuxc_gpr_base_regs *gpr =
-		(struct iomuxc_gpr_base_regs *)IOMUXC_GPR_BASE_ADDR;
-
-	/* Use 125M anatop REF_CLK1 for ENET1, not from external */
-	clrsetbits_le32(&gpr->gpr[1], 0x2000, 0);
-}
-
 int board_init(void)
 {
-	if (IS_ENABLED(CONFIG_FEC_MXC))
-		setup_fec();
-
 	return 0;
 }