Revert "mmc: davinci: drop struct davinci_mmc_plat"

Adam Ford reports that this change breaks booting on da850-evm and
Bartosz Golaszewski agrees that with the impending release we should
revert the change for now.  With that noted:

This reverts commit 21a4d80a.

Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Tested-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>