net: dsa: Remove hwmon support

Only the Marvell mv88e6xxx DSA driver made use of the HWMON support in
DSA. The temperature sensor registers are actually in the embedded
PHYs, and the PHY driver now supports it. So remove all HWMON support
from DSA and drivers.

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