net: phy: mxl-gpy: add temperature sensor

The GPY115 and GPY2xx PHYs contain an integrated temperature sensor. It
accuracy is +/- 5°C. Add support for it.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220622141716.3517645-1-michael@walle.cc
Signed-off-by: Jakub Kicinski <kuba@kernel.org>