mx31moboard: add sdhc support (v3)

This support both sdhc1 and sdhc2 with WP and CD for the marxbot and
devboard mx31moboard baseboards.

sdhc2 is present on both, but is not directly included in the
mx31moboard file because a third baseboard (not supported yet) without
sdhc2 is planned.

Changes since v1: removed pin initialization from init/exit function and
taken different comments into account

Changes since v2: pin initialiation now is done in another patch for all
current mx31moboard pins

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