net: atlantic: add hwmon getter for MAC temperature

This patch adds the possibility to obtain MAC temperature via hwmon.
On A1 there are two separate temperature sensors.
On A2 there's only one temperature sensor, which is used for reporting
both MAC and PHY temperature.

Signed-off-by: Mark Starovoytov <mstarovoitov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>