dt-bindings: mtd: Standardize the style in the examples

As recently requested by the binding maintaines, let's use 4 spaces in
the examples.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-18-miquel.raynal@bootlin.com

Documentation/devicetree/bindings/mtd/st,stm32-fmc2-nand.yaml

@@ -101,6 +101,7 @@ examples:
     #include <dt-bindings/interrupt-controller/arm-gic.h>
     #include <dt-bindings/clock/stm32mp1-clks.h>
     #include <dt-bindings/reset/stm32mp1-resets.h>
+
     nand-controller@58002000 {
         compatible = "st,stm32mp15-fmc2";
         reg = <0x58002000 0x1000>,