SAMA5 HSMCI: e-enable TX DMA and verify that DMA writes to the SD card are...

SAMA5 HSMCI: e-enable TX DMA and verify that DMA writes to the SD card are functional.  They are so now TX DMA is re-enabled in the driver.  This might affect the SAMA5D3 platforms where the TX DMA problem was found.  The SAMA4D3 and 4 use the same HSMCI driver.  Much has change since then and it is not surprising that DMA is now functional.  However, the has not be re-verified on the SAMA5D3 which has a different DMA controller.