pcf50633: introduces battery charging current control

Implement a new sysfs attribute to allow changing MBC charging limit on
the fly independently of usb current limit. It also gets set
automatically every time usb current limit is changed.

Limiting charging current also prevents violating USB specification in
the case when the whole device is shut down and usb current limit is
reset to the factory default by the pcf50633 state transition.

Signed-off-by: Balaji Rao <balajirrao@openmoko.org>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>