selftests/powerpc: Rename TARGETS in powerpc selftests makefile

This patch changes the name of the make variable TARGETS, to prevent it
from colliding with a value set by the user on the command line (as they
are recommended to do by tools/testing/selftests/README.txt).

Without this patch, "make -C tools/testing/selftests TARGETS=powerpc"
will fail.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>