hwmon: add TI ads7871 a/d converter driver

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Paul Thomas <pthomas8589@gmail.com>
Cc: Jonathan Cameron <jic23@cam.ac.uk>
Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>