net: add framework to support Ethernet PSE and PDs devices

This framework was create with intention to provide support for Ethernet PSE
(Power Sourcing Equipment) and PDs (Powered Device).

At current step this patch implements generic PSE support for PoDL (Power over
Data Lines 802.3bu) specification with reserving name space for PD devices as
well.

This framework can be extended to support 802.3af and 802.3at "Power via the
Media Dependent Interface" (or PoE/Power over Ethernet)

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>