net: phy: realtek: add helper RTL822X_VND2_C22_REG

C22 register space is mapped to 0xa400 in MMD VEND2 register space.
Add a helper to access mapped C22 registers.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/6344277b-c5c7-449b-ac89-d5425306ca76@gmail.com


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