dt-bindings: mmc: Add binding for BlueField SoC

This commit adds "mellanox,bluefield-dw-mshc" for dwmmc driver
extension on Mellanox BlueField SoC platform.

Signed-off-by: Liming Sun <lsun@mellanox.com>
Reviewed-by: David Woods <dwoods@mellanox.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>