spi: dt-bindings: tegra: Document IOMMU property for Tegra234 QSPI

Add the 'iommus' property to the Tegra QSPI device tree binding.
The property is needed for Tegra234 when using the internal DMA
controller, and is not supported on other Tegra chips, as DMA is
handled by an external controller.

Signed-off-by: Vishwaroop A <va@nvidia.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Link: https://patch.msgid.link/20250513200043.608292-1-va@nvidia.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/nvidia,tegra210-quad.yaml

@@ -10,9 +10,6 @@ maintainers:
   - Thierry Reding <thierry.reding@gmail.com>
   - Jonathan Hunter <jonathanh@nvidia.com>
 
-allOf:
-  - $ref: spi-controller.yaml#
-
 properties:
   compatible:
     enum:
@@ -47,6 +44,9 @@ properties:
       - const: rx
       - const: tx
 
+  iommus:
+    maxItems: 1
+
 patternProperties:
   "@[0-9a-f]+$":
     type: object
@@ -69,6 +69,18 @@ required:
 
 unevaluatedProperties: false
 
+allOf:
+  - $ref: spi-controller.yaml#
+  - if:
+      properties:
+        compatible:
+          not:
+            contains:
+              const: nvidia,tegra234-qspi
+    then:
+      properties:
+        iommus: false
+
 examples:
   - |
     #include <dt-bindings/clock/tegra210-car.h>