rivers/misc: add driver for Texas Instruments DAC7512

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: "H Hartley Sweeten" <hartleys@visionengravers.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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