arm64: dts: armada-3720-espressobin: Set mv88e6341 cpu port as RGMII-ID

The mv88e6341 ethernet switch needs the cpu port control register to be
set with TX and RX internal delay in order to work.

This fixes ethernet support on system booted via a bootloader that
has not already configured this register (e.g. mainline u-boot, or
vendor u-boot compiled without ethernet support).

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>