net: phy: realtek: Fix MMD access on RTL8126A-integrated PHY

All MMD reads return 0 for the RTL8126A-integrated PHY. Therefore phylib
assumes it doesn't support EEE, what results in higher power consumption,
and a significantly higher chip temperature in my case.
To fix this split out the PHY driver for the RTL8126A-integrated PHY
and set the read_mmd/write_mmd callbacks to read from vendor-specific
registers.

Fixes: 5befa372 ("net: phy: realtek: add support for RTL8126A-integrated 5Gbps PHY")
Cc: stable@vger.kernel.org
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>