bsps/xqspipsu: Handle SMP systems properly

The NOR driver was not written with SMP systems and caching in mind.
This makes the IsBusy flag volatile for updates across cores and
introduces cache flushing and invalidation where necessary for data
manipulated by the DMA engine in the QSPI peripheral.

bsps/include/dev/spi/xqspipsu.h

@@ -259,6 +259,9 @@ typedef struct {
 	s32 TxBytes;	 /**< Number of bytes to transfer (state) */
 	s32 RxBytes;	 /**< Number of bytes left to transfer(state) */
 	s32 GenFifoEntries;	 /**< Number of Gen FIFO entries remaining */
+#ifdef __rtems__
+	volatile
+#endif
 	u32 IsBusy;		 /**< A transfer is in progress (state) */
 	u32 ReadMode;		 /**< DMA or IO mode */
 	u32 GenFifoCS;		/**< Gen FIFO chip selection */