powerpc/powernv: Squash sparse warnings in opal-call.c

sparse complains a lot about opal-call.c:

  arch/powerpc/platforms/powernv/opal-call.c:128:1: warning: symbol 'opal_invalid_call' was not declared. Should it be static?
  arch/powerpc/platforms/powernv/opal-call.c:129:1: warning: symbol 'opal_console_write' was not declared. Should it be static?
  arch/powerpc/platforms/powernv/opal-call.c:130:1: warning: symbol 'opal_console_read' was not declared. Should it be static?

Those symbols are forward declared in opal.h, but we can't include that
because the function signatures in opal.h are different. So instead, just
add an extra forward declaration to the OPAL_CALL macro to shut sparse up.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

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

@@ -120,6 +120,8 @@ static int64_t opal_call(int64_t a0, int64_t a1, int64_t a2, int64_t a3,
 }
 
 #define OPAL_CALL(name, opcode)					\
+int64_t name(int64_t a0, int64_t a1, int64_t a2, int64_t a3,	\
+	     int64_t a4, int64_t a5, int64_t a6, int64_t a7);	\
 int64_t name(int64_t a0, int64_t a1, int64_t a2, int64_t a3,	\
 	     int64_t a4, int64_t a5, int64_t a6, int64_t a7)	\
 {								\