net: phy: adin: make RGMII internal delays configurable

The internal delays for the RGMII are configurable for both RX & TX. This
change adds support for configuring them via device-tree (or ACPI).

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>