iio: amplifiers: New driver for AD8366 Dual-Digital Variable Gain Amplifier

Changes since V1:

Apply review feedback:
	Introduce and use IIO_CHAN_INFO_HARDWAREGAIN
	Introduce and use Use IIO_VAL_INT_PLUS_MICRO_DB

Modify out of staging include paths.
Convert to new iio core API naming.

Changes since V2:

more sanity checking in write_raw

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>