tools/check_spacing: modularize and improve readability

This patch simplifies the way that `check_spacing.py` script works by
creating helper functions and reducing the number of nested for
loops.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: I9377bf4fa266dd9f66e18572e2ecff062d3eccc4