selftests/powerpc: Move shared headers into new include dir

There are some functions, especially register related, which can
be shared across multiple selftests/powerpc test directories.

This patch creates a new include directory to store those shared
files, so that the file layout becomes more neat.

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
[mpe: Reworked to move the headers only]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/stringloops/memcmp.c

@@ -1,7 +1,7 @@
 #include <malloc.h>
 #include <stdlib.h>
 #include <string.h>
-#include "../utils.h"
+#include "utils.h"
 
 #define SIZE 256
 #define ITERATIONS 10000