doc: Add clang-format configuration

This patch updates the code style documentation, adjusting it to fit existing established but undocumented rules, as well as introducing new ones for consistency.

It also introduces a clang-format configuration, which becomes the de jure code style arbitrator. This configuration uses a style very close to the existing style, with some minor tweaks in places where a single existing style was unestablished.

Change-Id: I7a6ae117a721d7e258df01a302c5a0fb18bc3133 Signed-off-by: Chris Kay chris.kay@arm.com