hwmon: (ibmpowernv) Use platform 'id_table' to probe the device

The current driver probe() function assumes the sensor device to be
always present and gets executed every time if the driver is loaded,
but the appropriate hardware could not be present.

So, move the platform device creation as part of platform init code
and use the 'id_table' to check if the device is present or not.

Signed-off-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>