net: dsa/mv88e6352: Report chip temperature

MV88E6352 supports reading the chip temperature from two PHY registers,
6:26 and 6:27. Report it using the more accurate register 6:27.
Also report temperature limit and alarm.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>