power: supply: twl4030-charger: add deferred probing for phy and iio

This fixes an issue if both this twl4030_charger driver and
phy-twl4030-usb are compiled as modules and loaded in random order.

It has been observed on GTA04 and OpenPandora devices that in worst
case the boot process hangs and in best case the AC detection fails
with a warning.

Therefore we add deferred probing checks for the usb_phy and the
iio channel for AC detection.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>