riscv: dts: microchip: add mac0 to mpfs-disco-kit dts

commit 9b00628062e646a26c2df6ab0578ccd57eba6e06 from
https://github.com/linux4microchip/linux.git

 linux-6.6-mchp+fpga

The Microchip PolarFire SoC Discovery kit dts has one ethernet port (J3).
Add ethernet support to the Microchip PolarFire SoC Discovery kit dts.

Signed-off-by: Valentina Fernandez <valentina.fernandezalanis@microchip.com>
Signed-off-by: harish h <harish.h@windriver.com>