net: mvpp2: implement pcs_inband_caps() method

Report the PCS in-band capabilities to phylink for Marvell PP2
interfaces.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1tIUsE-006IUh-E7@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>