iio: add Capella cm3218x ambient light sensor driver.

iio: add Capella cm3218x ambient light sensor driver.

Add Capella Microsystem CM3218X family Ambient Light Sensor IIO driver.
This driver will convert raw data to lux value.  Default parameters are
for reference only.  It will detect ACPI table to load per-system manufacturing
parameters.

V2: Follow Jonathan Cameron's advise to separate device id.
Also, follow Peter Meerwald's advise to correct whitespace, prefix, and,
comments.

V3: Follow Peter Meerwald's advise to create a separated i2c_client handler to
access SMBus ARA register.

V4: Follow Srinivas Pandruvada and Peter Meerwald advise.  Use i2c_transfer() to
access SMBus ARA register.

Signed-off-by: Kevin Tsai <ktsai@capellamicro.com>