mmc: dw_mmc-bluefield: Add driver extension

This commit adds extension to the dw_mmc driver for Mellanox BlueField
SoC. It updates the UHS_REG_EXT register to bring up the eMMC card on
this SoC.

Signed-off-by: Liming Sun <lsun@mellanox.com>
Reviewed-by: David Woods <dwoods@mellanox.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>