dma: edma: move device registration to platform code

The horrible split between the low-level part of the edma support
and the dmaengine front-end driver causes problems on multiplatform
kernels. This is an attempt to improve the situation slightly
by only registering the dmaengine devices that are actually
present.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[olof: add missing include of linux/dma-mapping.h]
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Olof Johansson <olof@lixom.net>