dt-bindings: mmc: ingenic: Support using bi-directional DMA channel

Update the binding documentation and the examples to support
bi-directional DMA channels.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20211220190840.108061-2-paul@crapouillou.net


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>