net: stmmac: add get_interfaces() platform method

Add a get_interfaces() platform method to allow platforms to indicate
to phylink which interface modes they support - which then allows
phylink to validate on initialisation that the configured PHY interface
mode is actually supported.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1uASLn-0021Qd-Mi@rmk-PC.armlinux.org.uk


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