dt-bindings: dma: uniphier-xdmac: switch to single reg region

The reg in the example "<0x5fc10000 0x1000>, <0x5fc20000 0x800>"
is wrong. The register region of this controller is much smaller,
and there is no other hardware register interleaved. There is no
good reason to split it into two regions.

Just use a single, contiguous register region.

While I am here, I made the 'dma-channels' property mandatory because
otherwise there is no way to determine the number of the channels.

Please note the original binding was merged recently. Since there
is no user yet, this change has no actual impact.

Fixes: b9fb56b6

 ("dt-bindings: dmaengine: Add UniPhier external DMA controller bindings")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200401032150.19767-1-yamada.masahiro@socionext.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>