mx6cuboxi: enable OF_CONTROL with DM_MMC and DM_USB

mx6cuboxi: enable OF_CONTROL with DM_MMC and DM_USB

Make first step in DT/DM migration. Enable OF_CONTROL only for the main
U-Boot image for now.

Remove mmc_init_main() because board_mmc_init() is not called when
DM_MMC is enabled.

DM_MMC requires DM_GPIO for card-detect to work. That in turn makes gpio
request mandatory. Add code to request/free gpios in platform code.

MMC devices are now numbered according to DT. The SD card is 1, and eMMC
is 2. Account for that in board_mmc_get_env_dev(), BOOT_TARGET_DEVICES,
and has_emmc().

DM_MMC requires BLK. However, the (BLK && !DM_USB) combination disables
USB_STORAGE. Enable DM_USB to preserve USB functionality. Add also
DM_REGULATORS for the USB power controller. This allows us to drop
board_ehci_hcd_init() and setup_usb().

Runtime selection of DT is necessary because of the i.MX6QD vs i.MX6SDL
incompatibility. DT selection does not rely on GPIOs, since DM_GPIO
depends on DT. Instead, we take one "fully featured" DT of each variant.
That should be enough to boot from both SD card and eMMC.

Since we don't select the exact DT, override the generic
show_board_info() that shows the selected DT 'model' field.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Fabio Estevam <festevam@gmail.com>