pda_power: add init and exit function callbacks

This adds init/exit function callbacks to pda_power, to
provide a place where the platform code can request/free
GPIOs that it wants to use in the is_ac_online, is_usb_online
and set_charge functions.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>