cmake: Add 'doc' target for doxygen documentation

The doc target generates the documentation for the project

independently of a PRODUCT and should be run with the command:

"make -f Makefile.cmake doc"

Signed-off-by: Tomás Agustín González Orlando <tomasagustin.gonzalezorlando@arm.com>
Change-Id: Ibcc3bc0a6935a52c73bf5c010880f50569f934c6