net: stmmac: introduce pcs_init/pcs_exit stmmac operations

Introduce a mechanism whereby platforms can create their PCS instances
prior to the network device being published to userspace, but after
some of the core stmmac initialisation has been completed. This means
that the data structures that platforms need will be available.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Co-developed-by: Romain Gantois <romain.gantois@bootlin.com>
Signed-off-by: Romain Gantois <romain.gantois@bootlin.com>
Reviewed-by: Hariprasad Kelam <hkelam@marvell.com>
Link: https://lore.kernel.org/r/20240513-rzn1-gmac1-v7-4-6acf58b5440d@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>