Split coding style documentation

Move into the code_rules.md new file the coding rules
that may have an impact on the compiled code. Keep in
code_style.md only the coding rules and guidelines
that don't have any impact on the compiled code.

Change-Id: Ie43a68966457f1ff821ba4fd95ef01f1001cf9cf
Signed-off-by: Ronald Cron <ronald.cron@arm.com>