selftests: Add intel_pstate to TARGETS

This test was missing from the TARGETS list.  The test requires patches
to cpupower to pass correctly.

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