This driver becomes the first user of the kernel's `ETHTOOL_PHY_EDPD`
phy-tunable feature.
EDPD is also enabled by default on PHY config_init, but can be disabled via
the phy-tunable control.

When enabling EDPD, it's also a good idea (for the ADIN PHYs) to enable TX
periodic pulses, so that in case the other PHY is also on EDPD mode, there
is no lock-up situation where both sides are waiting for the other to
transmit.

Via the phy-tunable control, TX pulses can be disabled if specifying 0
`tx-interval` via ethtool.

The ADIN PHY supports only fixed 1 second intervals; they cannot be
configured. That is why the acceptable values are 1,
ETHTOOL_PHY_EDPD_DFLT_TX_MSECS and ETHTOOL_PHY_EDPD_NO_TX (which disables
TX pulses).
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>