net: stmmac: add select_pcs() platform method

Allow platform drivers to provide their logic to select an appropriate
PCS.

Tested-by: Romain Gantois <romain.gantois@bootlin.com>
Reviewed-by: Romain Gantois <romain.gantois@bootlin.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1sHhoM-00Fesu-8E@rmk-PC.armlinux.org.uk


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