docs: dt: writing-schema: mention coding style

Mention the usage of YAML coding style. Describe explicitly that
four-space indentation in DTS examples is preferred, because:
1. The YAML's default two-space indentation for DTS code makes it
significantly less readable.
2. Linux coding style tabs would introduce inconsistency (entire file is
indented with spaces).
3. On the other hand, eight spaces would not align with example's
opening ' - |' part. Four spaces makes the code nicely aligned with
it.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/example-schema.yaml

···
   # be overridden or an appropriate parent bus node should be shown (such as on
   # i2c buses).
   #
-  # Any includes used have to be explicitly included.
+  # Any includes used have to be explicitly included. Use 4-space indentation.
   - |
     node@1000 {
-          compatible = "vendor,soc4-ip", "vendor,soc1-ip";
-          reg = <0x1000 0x80>,
-                <0x3000 0x80>;
-          reg-names = "core", "aux";
-          interrupts = <10>;
-          interrupt-controller;
+        compatible = "vendor,soc4-ip", "vendor,soc1-ip";
+        reg = <0x1000 0x80>,
+              <0x3000 0x80>;
+        reg-names = "core", "aux";
+        interrupts = <10>;
+        interrupt-controller;
     };

Documentation/devicetree/bindings/writing-schema.rst

···
 values a matrix (in order to define groupings) even when only a single value
 is present. Single entries in schemas are fixed up to match this encoding.
 
+Coding style
+------------
+
+Use YAML coding style (two-space indentation). For DTS examples in the schema,
+preferred is four-space indentation.
+
 Testing
 -------