hwmon: (adm1275) Enable adm1278 VOUT sampling

The adm1278 can optionally monitor the VOUT pin. This functionality is
not enabled at reset, so PMON_CONFIG needs to be modified in order to
enable it.

Signed-off-by: Yi Li <adamliyi@msn.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>