powerpc: Move `path` variable inside DEBUG_PROM

Add gcc attribute unused for two variables. Fix warnings treated as errors
with W=1:

  arch/powerpc/kernel/prom_init.c:1388:8: error: variable ‘path’ set but not used [-Werror=unused-but-set-variable]
Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/prom_init.c

@@ -628,7 +628,7 @@ static void __init early_cmdline_parse(void)
 	const char *opt;
 
 	char *p;
-	int l = 0;
+	int l __maybe_unused = 0;
 
 	prom_cmd_line[0] = 0;
 	p = prom_cmd_line;
@@ -1421,7 +1421,10 @@ static void __init reserve_mem(u64 base, u64 size)
 static void __init prom_init_mem(void)
 {
 	phandle node;
-	char *path, type[64];
+#ifdef DEBUG_PROM
+	char *path;
+#endif
+	char type[64];
 	unsigned int plen;
 	cell_t *p, *endp;
 	__be32 val;
@@ -1442,7 +1445,9 @@ static void __init prom_init_mem(void)
 	prom_debug("root_size_cells: %x\n", rsc);
 
 	prom_debug("scanning memory:\n");
+#ifdef DEBUG_PROM
 	path = prom_scratch;
+#endif
 
 	for (node = 0; prom_next_node(&node); ) {
 		type[0] = 0;