scripts: check-sysctl-docs: adapt to new API

The script expects the old sysctl_register_paths() API which was removed
some time ago. Adapt it to work with the new
sysctl_register()/sysctl_register_sz()/sysctl_register_init() APIs.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Joel Granados <j.granados@samsung.com>