dt-bindings: mmc: synopsys-dw-mshc: add iommus for Intel SocFPGA

The DW MSHC node in Intel SocFPGA ARM64 DTS has iommus property, so
allow it to silence dtbs_check warnings:

socfpga_n5x_socdk.dtb: mmc@ff808000: Unevaluated properties are not allowed ('iommus' was unexpected)

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

Documentation/devicetree/bindings/mmc/synopsys-dw-mshc.yaml

···
       - const: biu
       - const: ciu
 
+  iommus:
+    maxItems: 1
+
   altr,sysmgr-syscon:
     $ref: /schemas/types.yaml#/definitions/phandle-array
     items:
···
         altr,sysmgr-syscon: true
     else:
       properties:
+        iommus: false
         altr,sysmgr-syscon: false
 
 required: