mtd: rawnand: sunxi: Add MDMA support

This patch enables NAND MDMA (MBUS DMA) mode for
the Allwinner SoCs A23/A33/H3.

The DMA transfer method gets sets now to MBUS DMA as default for
the sun8i-a23-nand-controller (till now DMA transfer was executed
via the shared DMA engine).

The main advantage is more bandwidth for the users of the shared DMA
engine and also that the MBUS DMA setup requires less configuration
effort. For example you don't need to define a dedicated DMA channel
in the device-tree any more.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Manuel Dipolt <manuel.dipolt@robart.cc>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/154840787.280672.1602517282173.JavaMail.zimbra@robart.cc