arm64: dts: freescale: imx8mp-var-som: Add EQoS support with MaxLinear PHY

arm64: dts: freescale: imx8mp-var-som: Add EQoS support with MaxLinear PHY

Enable the EQoS Ethernet controller on the i.MX8MP VAR-SOM with the
integrated Maxlinear MXL86110 PHY. The PHY is connected to the EQOS
MDIO bus at address 4.

This patch adds:
- EQOS controller configuration with RGMII interface.
- Proper reset timings.
- PHY power supply regulators.
- RGMII pinmux configuration for all data, control and clock signals.
- LED configuration for link status indication via the LED subsystem
  under /sys/class/leds/, leveraging the support implemented in the.
  mxl86110 PHY driver (drivers/net/phy/mxl-86110.c).
  Two LEDs are defined to match the LED configuration on the Variscite
  VAR-SOM Carrier Boards:
    * LED@0: Yellow, netdev trigger.
    * LED@1: Green, netdev trigger.

The RGMII TX/RX delays are implemented in SOM via PCB passive
delays, so no software delay configuration is required.

Signed-off-by: Stefano Radaelli <stefano.radaelli21@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>