net: phy: adin: make RMII fifo depth configurable

The FIFO depth can be configured for the RMII mode. This change adds
support for doing this 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>