dt-bindings: mmc: meson-gx: add optional power-domains

On newer SoCs, the MMC controller can require a power-domain to operate,
add it as optional.

Signed-off-by: Neil Armstrong <[e-mail]>
Acked-by: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/20240605-topic-aml[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

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

···
       set when controller's internal DMA engine cannot access the DRAM memory,
       like on the G12A dedicated SDIO controller.
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg