dmaengine i.MX SDMA: reserve channel 0 by not registering it

We need channel 0 of the sdma engine for internal purposes. We
accomplished this by calling dma_request_channel() in the probe
function. This does not work when multiple dma engines are
present which is the case when IPU support for i.MX31/35 is
compiled in. So instead of registering channel 0 and reserving
it afterwards simply do not register it in the first place.
With this the dmaengine channel counting does not match sdma
channel counting anymore, so we have to use sdma channel counting
in the driver.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>