net: pse-pd: add regulator based PSE driver

Add generic, regulator based PSE driver to support simple Power Sourcing
Equipment without automatic classification support.

This driver was tested on 10Bast-T1L switch with regulator based PoDL PSE.

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