blackfin: fix a warning in arch/blackfin/cpu/cpu.c

This commit fixes:
    cpu.c:107: warning: ‘noreturn’ function does return

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/arch/blackfin/cpu/cpu.c b/arch/blackfin/cpu/cpu.c
index 218f57e..2409c30 100644
--- a/arch/blackfin/cpu/cpu.c
+++ b/arch/blackfin/cpu/cpu.c
@@ -104,6 +104,9 @@
 
 	serial_early_puts("Board init flash\n");
 	board_init_f(bootflag);
+
+	/* should not be reached */
+	while (1);
 }
 
 int exception_init(void)