dt-bindings: mmc: meson-gx: add dram-access-quirk property

On the Amlogic G12A SoC family, (only) the SDIO controller has a bug which
makes any DRAM access from the MMC controller fail.

Add the amlogic,dram-access-quirk property so signal this particular
controller has this bug and needs a quirk to work properly.
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/amlogic,meson-gx.txt

@@ -22,6 +22,10 @@ Required properties:
   clock rate requested by the MMC core.
 - resets     : phandle of the internal reset line
 
+Optional properties:
+- amlogic,dram-access-quirk: set when controller's internal DMA engine cannot access the
+  DRAM memory, like on the G12A dedicated SDIO controller.
+
 Example:
 
 	sd_emmc_a: mmc@70000 {