include/devicetree: dma: add DT_DMAS_CELL_BY_NAME_OR macro

Add the DT_DMAS_CELL_BY_NAME_OR macro. It is almost the same as
DT_DMAS_CELL_BY_NAME, but it uses DT_PHA_BY_NAME_OR instead of
DT_PHA_BY_NAME.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>