mx31ads: Fix build for missing mx31.h

Several of the macros in mx31ads.h depend on mx31.h which is no longer
included in quite so many standard headers as it once was. Include it
directly so we can build.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>