dt-bindings: dma: uart: rename binding

The filename matches mtk-uart-apdma.c.
So using "mtk-uart-apdma.txt" should be better.
And add some property.

Signed-off-by: Long Cheng <long.cheng@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>