powerpc: Introduce asm-prototypes.h

Sparse picked up a number of functions that are implemented in C and
then only referred to in asm code.

This introduces asm-prototypes.h, which provides a place for
prototypes of these functions.

This silences some sparse warnings.

Signed-off-by: Daniel Axtens <dja@axtens.net>
[mpe: Add include guards, clean up copyright & GPL text]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/opal-tracepoints.c

@@ -1,6 +1,7 @@
 #include <linux/percpu.h>
 #include <linux/jump_label.h>
 #include <asm/trace.h>
+#include <asm/asm-prototypes.h>
 
 #ifdef HAVE_JUMP_LABEL
 struct static_key opal_tracepoint_key = STATIC_KEY_INIT;