scripts/coccinelle: Add Coccinelle script for unsigned values

Add a Coccinelle script that adds an 'U' to values assigned to
unsigned variables, according ot MISRA-C rule 7.2.

Add a 'report' mode to the script that can be used by developer/CI
and a 'patch' mode that should do the heavy lifting.

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