net: stmmac: provide PCS initialisation hook

net: stmmac: provide PCS initialisation hook

dwmac cores provide a feature bit to indicate when the PCS block is
present, but features are only read after the core's setup() function
has been called, meaning we can't decide whether to initialise the
integrated PCS in the setup function. Provide a new MAC core hook
for PCS initialisation, which will be called after the feature
registers have been read.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://patch.msgid.link/E1v9P6x-0000000AomL-3OKd@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>