net: phy: realtek: add support for reading MDIO_MMD_VEND2 regs on RTL8125/RTL8126

net: phy: realtek: add support for reading MDIO_MMD_VEND2 regs on RTL8125/RTL8126

RTL8125/RTL8126 don't support MMD access to the internal PHY, but
provide a mechanism to access at least all MDIO_MMD_VEND2 registers.
By exposing this mechanism standard MMD access functions can be used
to access the MDIO_MMD_VEND2 registers.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/e821b302-5fe6-49ab-aabd-05da500581c0@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>