hwmon: ADC124S501 generic driver

SPI driver for analog to digital converters national semiconductor
ADC081S101, ADC124S501, ...

Code for 8 channels by Tobias Himmer.

This driver adds support for National Semiconductor ADC<bb><c>S<sss> chip
family, where:

 * bb  is the resolution in number of bits (8, 10, 12)
 * c   is the number of channels (1, 2, 4, 8)
 * sss is the maximum conversion speed (021 for 200 kSPS, 051 for 500
   kSPS and 101 for 1 MSPS)

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Marc Pignat <marc.pignat@hevs.ch>
Cc: Tobias Himmer <tobias@himmer-online.de>
Cc: "Mark M. Hoffman" <mhoffman@lightlink.com>
Cc: Jean Delvare <khali@linux-fr.org>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>