dra7: Allow selecting a new dtb after board detection.

dra7: Allow selecting a new dtb after board detection.

The DRA7 platforms requires that the dtb used in the SPL really matches the
platform  to have the best MMC performances.
To detect the board type/version an I2C EEPROM is read. This requires that
DM is initialized before the detection. As a consequence we must reset the
DM after the board detection is a new dtb would better match the platform.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>