mmc: jz4740: Support using a bi-directional DMA channel

mmc: jz4740: Support using a bi-directional DMA channel

Since the MMC/SD controller in Ingenic SoCs work in half-duplex, it is
possible to use one single DMA channel for both TX and RX operations,
instead of using separate channels.

As some older Ingenic SoCs offer only a handful of DMA channels,
supporting bi-directional channels allow more hardware to use the
channels that would otherwise be used for the MMC/SD operation.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20211220190840.108061-3-paul@crapouillou.net
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>