ata: arasan: remove the need for platform_data

This adds a complete DT binding for the arasan device driver. There is
currently only one user, which is the spear13xx platform, so we don't
actually have to parse all the properties until another user comes in,
but this does use the generic DMA binding to find the DMA channel.

The patch is untested so far and is part of a series to convert
the spear platform over to use the generic DMA binding, so it
should stay with the rest of the series.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.linux@linaro.org>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Jeff Garzik <jgarzik@redhat.com>
Cc: devicetree-discuss@lists.ozlabs.org