FDT: Add compatible string for DWMMC

Add required compatible information for DWMMC driver.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Amar <amarendra.xt@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>