staging:iio: Add channel types IIO_CURRENT and IIO_POWER.

This is required for the ADE7758 driver cleanup.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>