mmc: sdhci: Move DMA handling to prepare_dma() function

In preparation for addition of ADMA2 support, cleanup SDMA handling by
moving it to a new sdhci_prepare_dma() function. Also add a flags field
in sdhci_host to indicate if DMA is enabled.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>

include/sdhci.h

@@ -272,6 +272,9 @@ struct sdhci_host {
 	uint	voltages;
 
 	struct mmc_config cfg;
+	dma_addr_t start_addr;
+	int flags;
+#define USE_SDMA	(0x1 << 0)
 };
 
 #ifdef CONFIG_MMC_SDHCI_IO_ACCESSORS