[BATTERY] pda_power platform driver

Common power driver for PDAs and phones with one or two external
power supplies (AC/USB) connected to main and backup batteries,
and optional builtin charger.

It's used to stop logic duplication through different embedded
devices. So, power supply *logic* is here. pda_power register
power supplies, and will take care about notifying batteries
about power changes through external power interface.

Currently, power consumption legal limits (including USB power
consumption) should be handled by platform code, inside set_charge
function.

Signed-off-by: Anton Vorontsov <cbou@mail.ru>
Signed-off-by: Roman Moravcik <roman.moravcik@gmail.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>