power: supply: axp20x_usb_power: add IIO dependency

When CONFIG_IIO=m and the axp20x_usb_power driver is built-in, we get
a link time error:

drivers/power/built-in.o: In function `axp20x_usb_power_get_property':
undefined reference to `iio_read_channel_processed'
drivers/power/built-in.o: In function `axp20x_usb_power_probe':
undefined reference to `devm_iio_channel_get'
undefined reference to `devm_iio_channel_get'

This adds the same dependency that we already have for the AC power driver
to the USB power driver. For consistency, I'm also moving the two closer
together in the Kconfig file.

Fixes: 33863c93 ("power: supply: axp20x_usb_power: use IIO channels when available")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>