selftests/intel_pstate: Update makefile to match new style

Recent changes from Bamvor (88baa78d

) have standardized the
variable names like TEST_GEN_FILES and removed the need for make targets
all and clean.

These changes bring the intel_pstate test inline with those changes.

Cc: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>