spi: fspi: Add power-domains to the DT bindings

i.MX8(X) based SoC use a power domain. Allow supplying this domain in
bindings.

Signed-off-by: Alexander Stein <[e-mail]>
Acked-by: Han Xu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/spi-nxp-fspi.yaml

···
       - const: fspi_en
       - const: fspi
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg