Add clang-tidy support and fix warnings

To be able to run clang-tidy in the linux driver stack, a clang-tidy
config file has been added and the cmake toolchain files have been
updated to generate a compile command database.

Issues and style inconsistencies reported by clang-tidy have been fixed.

Change-Id: I003c751b3eaf0527974a644a2989b24d41788796
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>