net: dsa: Centralise code for reading the temperature sensor

The method to read the temperature used in the mve6123_61_65 driver
can also be used for other chips. Move the code into the shared code
base of mv88e6xxx.c.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>