In some cases (such as a boot from SPI) the bootloader or the ROM code may
leave a bias pull-down on the mmc pins. If so the MMC will fail during the
initialisation.

Explicitly disabling the pinmux solves the problem.
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>