mmc: davinci: fix mmc boot in SPL

The MMC boot is currently broken on omapl138-lcdk after enabling the
driver model in SPL. The main problem is the driver's bind callback not
being called after probe in SPL (even with the DM_FLAG_PRE_RELOC flag
specified).

While a proper fix is still being worked on, this hacky changeset at
least fixes the MMC boot on this platform by calling mmc_bind()
manually from probe().

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>