dmaengine: dma-jz4780: Return error if not probed from DT

The driver calls clk_get() with the clock name set to NULL, which means
that the driver could only work when probed from devicetree. From now
on, we explicitly require the driver to be probed from devicetree.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>