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>

arch/blackfin/cpu/cpu.c

@@ -104,6 +104,9 @@ void cpu_init_f(ulong bootflag, ulong loaded_from_ldr)
 
 	serial_early_puts("Board init flash\n");
 	board_init_f(bootflag);
+
+	/* should not be reached */
+	while (1);
 }
 
 int exception_init(void)