CODING_STYLE: Custom type names must be snake-cased by word

There's the unwritten convention of splitting type names using
underscores. Add such convention to the CODINNG_STYLE to make it
common and less unwritten.

Signed-off-by: Alejandro Vallejo <agarciav@amd.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

CODING_STYLE

@@ -113,6 +113,12 @@ Braces should be omitted for blocks with a single statement. e.g.,
 if ( condition )
     single_statement();
 
+Identifiers
+-----------
+
+When giving names to identifiers (variables, functions, constants...), separate
+words using underscores (for example, use "my_foo" instead of "myfoo").
+
 Types
 -----