mx31moboard: initial support for various baseboards

This enables our mx31moboard to be used on the different baseboards that
we are developping according to the application needs. There are not
many differences between the boards for now, but when other peripherals
are available for mx31 the differences are going to grow.

v2: takes Sascha's comments into account

Signed-off-by: Valentin Longchamp <valentin.longchamp@epfl.ch>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>