mmc: dwmmc: socfpga: Add ATF support for MMC driver

In non-secure mode (EL2), MMC driver calls the SMC/PSCI services
provided by ATF to set SDMMC's DRVSEL and SMPLSEL.

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>