dt-bindings: misc: explicitly add #address-cells for slave mode

Explicitly add "#address-cells = <0>" and "#size-cells = <0>" to
eliminate below warnings.

(spi_bus_bridge): /example-0/spi: incorrect #address-cells for SPI bus
(spi_bus_bridge): /example-0/spi: incorrect #size-cells for SPI bus
(spi_bus_reg): Failed prerequisite 'spi_bus_bridge'
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20201013160845.1772-5-thunder.leizhen@huawei.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/misc/olpc,xo1.75-ec.yaml

@@ -42,6 +42,8 @@ examples:
 
     spi {
       spi-slave;
+      #address-cells = <0>;
+      #size-cells = <0>;
       ready-gpios = <&gpio 125 GPIO_ACTIVE_HIGH>;
 
       slave {