dt-bindings: writing-schema: add example for multiple DT_SCHEMA_FILES

Variable DT_SCHEMA_FILES can specify multiple files or patterns, but the
documentation does not indicate how to do it. Each entry needs to be
separated by a column symbol ':'.

Add an example to make it more obvious how to specify multiple
DT_SCHEMA_FILES.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20231002183813.1702052-1-hugo@hugovil.com


Signed-off-by: Rob Herring <robh@kernel.org>