dt-bindings: spi: spi-cadence: Add label property

Add a label property to allow a custom name to be used for identifying the
controller on a board. This is useful when there is more than one
controller present. Similar change was done by commit 412b7a521c30
("dt-bindings: eeprom: at24: Add label property for AT24").

Signed-off-by: Michal Simek <[e-mail]>
Link: https://lore.kernel.org/r/82cd[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/spi-cadence.yaml

···
   power-domains:
     maxItems: 1
 
+  label:
+    description: Descriptive name of the SPI controller.
+
 required:
   - compatible
   - reg