cmake: Add Clang-Tidy support

Clang-Tidy is a static analyzer that exists as part of the LLVM/Clang
project. Similarly to CPPCheck, it is invoked as part of the build
process on individual source files.

Change-Id: I68c98540eefc2b547f86d9fc9838ea5de2c06f05
Signed-off-by: Chris Kay <chris.kay@arm.com>
Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>