dmaengine: hdmac: Implement interleaved transfers

The AT91 HDMAC controller supports interleaved transfers through what's
called the Picture-in-Picture mode, which allows to transfer a squared
portion of a framebuffer.

This means that this interleaved transfer only supports interleaved
transfers which have a transfer size and ICGs that are fixed across all the
chunks.

While this is a quite drastic restriction of the interleaved transfers
compared to what the dmaengine API allows, this is still useful, and our
driver will only reject transfers that do not conform to this.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
diff --git a/drivers/dma/at_hdmac_regs.h b/drivers/dma/at_hdmac_regs.h
index 2727ca5..bc8d5eb 100644
--- a/drivers/dma/at_hdmac_regs.h
+++ b/drivers/dma/at_hdmac_regs.h
@@ -196,6 +196,11 @@
 	size_t				len;
 	u32				tx_width;
 	size_t				total_len;
+
+	/* Interleaved data */
+	size_t				boundary;
+	size_t				dst_hole;
+	size_t				src_hole;
 };
 
 static inline struct at_desc *