tools/check_style: introduce code style check script

This commit introduces `check_style.py` into the tools directory.
It checks code style against our standards using `git clang-format`.

Key features include:
- Option to specify an output file (default: 'code-style.patch').
- Automatic retrieval of the previous commit's hash.
- Generates a patch and provides instructions if deviations are found.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: I144723eb270fa7b8fc7f94a718ef461258c77dbf