net: lan743x: Add support to SGMII 1G and 2.5G

Add SGMII access read and write functions
Add support to SGMII 1G and 2.5G for PCI11010/PCI11414 chips

Signed-off-by: Raju Lakkaraju <Raju.Lakkaraju@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>