pm-graph: Update directory handling and installation process in Makefile

 - Standardize directory variables to support more flexible installations.
 - Add copyright and licensing information to the Makefile.
 - Introduce ".PHONY" declarations to ensure that specific targets are always
   executed, regardless of the presence of files with matching names.
 - Add a help target to provide usage instructions.

Signed-off-by: Amit Vadhavana <av2082000@gmail.com>
Acked-by: Todd Brandt <todd.e.brandt@linux.intel.com>
Link: https://patch.msgid.link/Update

 directory handling and installation process in Makefile
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>