net/smsc911x: Add minimal runtime PM support

Add minimal runtime PM support (enable on probe, disable on remove), to
ensure proper operation with a parent device that uses runtime PM.

This is needed on systems where the external bus controller module of
the SoC is contained in a PM domain and/or has a gateable functional
clock. In such cases, before accessing any device connected to the
external bus, the PM domain must be powered up, and/or the functional
clock must be enabled, which is typically handled through runtime PM by
the bus controller driver.

An example of this is the kzm9g development board, where an smsc9220
Ethernet controller is connected to the Bus State Controller (BSC) of a
Renesas sh73a0 SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>