scripts/coccinelle: Add script for counting identifier length

Add a simple Coccinelle script that counts identifier lengths and
prints out a warning if it is longer than 31 characters.

The script can be run with:
spatch -D report --very-quiet \
--include-headers --recursive-includes \
--cocci-file $ZEPHYR_BASE/scripts/coccinelle/identifier_length.cocci \
--dir $ZEPHYR_BASE \
kernel/

Where '--include-headers' and '--recursive-includes' can be omitted
if neede.

Signed-off-by: Patrik Flykt <patrik.flykt@intel.com>