max8925_power: Check at probe time if power to set online

The interrupt for ac on/off can be missed during boot time.
Check if online by seeing if we have power.  We choose 0.5V
since this is high enough to avoid random reading from a
input that could be floating if no charger.

Signed-off-by: Philip Rakity <prakity@marvell.com>
Signed-off-by: Ted Bennett <tbennett@marvell.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>