net: stmmac: Support a generic PCS field in mac_device_info

Global stmmac support for early initialization of PCS devices requires a
generic PCS reference that can be passed to phylink_pcs_pre_init().
Currently, the mac_device_info struct contains only one PCS field, which is
specific to the Lynx model.

As PCS models are hardware-specific, it is more appropriate to have a
generic PCS field in the mac_device_info struct.

Refactor the lynx_pcs field into a generic phylink_pcs field.

Signed-off-by: Romain Gantois <romain.gantois@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240326-rxc_bugfix-v6-4-24a74e5c761f@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>