staging: iio: TAOS tsl258x: Device driver

Added suspend/resume functions.
Changed attribute names to match existing where applicable and updated
or documented new ABI as discussed.
Changed integration time ABI from using index (0 to 3) to use actual
gain values (1x,8x, etc.).
Removed various unused variables, declarations, and functions.
Revised code to accommodate different endianess (le16_to_cpu).
Updated error return codes in various functions.
Changed from mdelay to msleep after determining that longer wait would
be acceptable.

V5: Makefile and Kconfig cleanups by Jonathan Cameron.

Signed-off-by: Jon August Brenner <jbrenner@taosinc.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>