microblaze: Fix sparse warning - prom.c

Warning log:
CHECK   arch/microblaze/kernel/dma.c
arch/microblaze/kernel/dma.c:53:26: warning: incorrect type in argument 1 (different base types)
arch/microblaze/kernel/dma.c:53:26:    expected int [signed] gfp
arch/microblaze/kernel/dma.c:53:26:    got restricted unsigned int [usertype] flag
Signed-off-by: NMichal Simek <monstr@monstr.eu>

arch/microblaze/kernel/prom.c

@@ -59,7 +59,7 @@ static int __init early_init_dt_scan_serial(unsigned long node,
 {
 	unsigned long l;
 	char *p;
-	int *addr;
+	const __be32 *addr;
 
 	pr_debug("search \"serial\", depth: %d, uname: %s\n", depth, uname);