net: phy: realtek: HWMON support for standalone versions of RTL8221B and RTL8251

HWMON support has been added for the RTL8221/8251 PHYs integrated together
with the MAC inside the RTL8125/8126 chips. This patch extends temperature
reading support for standalone variants of the mentioned PHYs.

I don't know whether the earlier revisions of the RTL8226 also have a
built-in temperature sensor, so they have been skipped for now.

Tested on RTL8221B-VB-CG.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>