dma: of: Remove restriction that #dma-cells can't be 0

There is no sensible reason why #dma-cells shouldn't be allowed to be 0. It is
completely up to the DMA controller how many additional parameters, besides the
phandle, it needs to identify a channel. E.g. for DMA controller with only one
channel or for DMA controllers which don't have a restriction on which channel
can be used for which peripheral it completely legitimate to not require any
additional parameters.

Also fixes the following warning:
	drivers/dma/of-dma.c: In function 'of_dma_controller_register':
	drivers/dma/of-dma.c:67:7: warning: 'nbcells' may be used uninitialized in this function

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>