iio: buffer-dma: sync upstream and add mmap legacy kconfig

Bring the buffer-dma interface in sync (as possible) with upstream. On
top of it, the MMAP support is under a new kconfig flag (which for now
is on by default) and can be opted out in favor of the new DMABUF
interface.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>

Author: nunojsa

drivers/iio/buffer/Kconfig

@@ -30,6 +30,11 @@ config IIO_BUFFER_DMAENGINE
 
 	  Should be selected by drivers that want to use this functionality.
 
+config IIO_DMA_BUF_MMAP_LEGACY
+	bool "Enables I/O DMA buffer legacy MMAP support"
+	depends on IIO_BUFFER_DMAENGINE
+	default y
+
 config IIO_BUFFER_HW_CONSUMER
 	tristate "Industrial I/O HW buffering"
 	help