net: phy: realtek: add 5Gbps support to rtl822x_config_aneg()

RTL8126 as an evolution of RTL8125 supports 5Gbps. rtl822x_config_aneg()
is used by the PHY driver for the integrated PHY, therefore add 5Gbps
support to it.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/5644ab50-e3e9-477c-96db-05cd5bdc2563@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>