dmaengine: sun4i: Add support for the DMA engine on sun[457]i SoCs

This patch adds support for the DMA engine present on Allwinner A10,
A13, A10S and A20 SoCs. This engine has two kinds of channels: normal
and dedicated. The main difference is in the mode of operation;
while a single normal channel may be operating at any given time,
dedicated channels may operate simultaneously provided there is no
overlap of source or destination.

Hardware documentation can be found on A10 User Manual (section 12), A13
User Manual (section 14) and A20 User Manual (section 1.12)

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>