board: tx48: add MMC init code

board: tx48: add MMC init code

Up to now we relied on cpu_mmc_init() to initialize the MMC unit for
us. This had some major drawbacks:
- the default implementation doesn't use CD gpios, so that the driver
  always assumes a card is present.
- the default implementation always initializes two MMC interfaces,
  though the TX48 supports only one.

Properly initialize the CD GPIO and implement our own version of
cpu_mmc_init().