board/BuR/brxre1: fix MMC boot

since commit

'd5abcf94' -
ti: boot: Register the MMC controllers in SPL in the same way as in u-boot

MMC boot on brxre1 board is broken, with this commit we make our board
working again.

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jean-Jacques Hiblot <jjhiblot@ti.com>