dmaengine: imx-sdma: add virt-dma support

The legacy sdma driver has below limitations or drawbacks:
  1. Hardcode the max BDs number as "PAGE_SIZE / sizeof(*)", and alloc
     one page size for one channel regardless of only few BDs needed
     most time. But in few cases, the max PAGE_SIZE maybe not enough.
  2. One SDMA channel can't stop immediatley once channel disabled which
     means SDMA interrupt may come in after this channel terminated.There
     are some patches for this corner case such as commit "2746e2c3",
     but not cover non-cyclic.

The common virt-dma overcomes the above limitations. It can alloc bd
dynamically and free bd once this tx transfer done. No memory wasted or
maximum limititation here, only depends on how many memory can be requested
from kernel. For No.2, such issue can be workaround by checking if there
is available descript("sdmac->desc") now once the unwanted interrupt
coming. At last the common virt-dma is easier for sdma driver maintain.
Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/Kconfig

@@ -250,6 +250,7 @@ config IMX_SDMA
 	tristate "i.MX SDMA support"
 	depends on ARCH_MXC
 	select DMA_ENGINE
+	select DMA_VIRTUAL_CHANNELS
 	help
 	  Support the i.MX SDMA engine. This engine is integrated into
 	  Freescale i.MX25/31/35/51/53/6 chips.