pcf50633: Add ac power supply class to the charger

This adds an appropriate ac power_supply class and shows usb only when
at the appropriate current limit.

Signed-off-by: Sean McNeil <sean@mcneil.com>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>