Currently, the .port_set_rgmii_delay hook is missing for the 88E6320
family, which causes failure to retrieve an IP address via DHCP.

Add mv88e6320_port_set_rgmii_delay() that allows applying the RGMII
delay for ports 2, 5, and 6, which are the only ports that can be used
in RGMII mode.

Tested on a custom i.MX8MN board connected to an 88E6320 switch.

This change also applies safely to the 88E6321 variant.

The only difference between 88E6320 versus 88E6321 is the temperature
grade and pinout.

They share exactly the same MDIO register map for ports 2, 5, and 6,
which are the only ports that can be used in RGMII mode.
Signed-off-by: Steffen Bätz <steffen@innosonix.de>
[fabio: Improved commit log and extended it to mv88e6321_ops]
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20221028163158.198108-1-festevam@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>