mmc: dw_mmc: Add support DW SD/MMC driver on SOCFPGA

Add platform specific functionality for the DW SD/MMC driver for
SoCFPGA. Move SDMMC_CMD_USE_HOLD_REG to dw_mmc.h so other platforms
can use this define.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Reviewed-by: Pavel Machek <pavel@denx.de>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>