sd_card & imx53: flush ADMA2 writes via DSB

We have to issue a data synchronization barrier after writing a ADMA2
table to ensure that the corresponding write commands were actually
executed before issuing the SD command.

Ref #1497

repos/os/src/drivers/sd_card/spec/imx53/esdhcv2.h

@@ -112,6 +112,8 @@ namespace Adma2
 					_base_virt[index] = desc;
 					consumed += curr;
 				}
+				/* ensure that all descriptor writes were actually executed */
+				asm volatile ("dsb");
 				return true;
 			}