board: colibri-imx6ull: fix detecting sd card

board: colibri-imx6ull: fix detecting sd card

Turns out on certain carrier boards (e.g. Iris V2) and under certain
circumstances (e.g. after a software reset) the SD card may have been
left in a strange state which later failed as follows:

Colibri iMX6ULL # mmc dev 0
Card did not respond to voltage select! : -110

Fix this as follows:
- Re-name the signaling voltage rail regulator from vmmc to vqmmc.
- Fix the name of the GPIO property to gpios.
- Specify 4-bit bus width, no write-protect capability and no 1.8
  volt signaling voltage capability.
- Fix the clock vs. command pull-up vs. push-pull configuration.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>