net: stmmac: add support for RZ/N1 GMAC

Add support for the Renesas RZ/N1 GMAC. This support can make use of a
custom RZ/N1 PCS which is fetched by parsing the pcs-handle device tree
property.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Co-developed-by: Romain Gantois <romain.gantois@bootlin.com>
Signed-off-by: Romain Gantois <romain.gantois@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Hariprasad Kelam <hkelam@marvell.com>
Link: https://lore.kernel.org/r/20240513-rzn1-gmac1-v7-6-6acf58b5440d@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>