dts: arm: renesas: ra: ra6: add Ethernet support

Define Ethernet and MDIO nodes in the RA6 Cortex-M33 SoC device tree
files in a similar manner to the RA8 SoC device tree files.

Removed those nodes from R6E2 SoC as it does not have Ethernet support.

Signed-off-by: Martino Facchin <m.facchin@arduino.cc>