libc/printf: add support to print "%" character

Enable printf() in TF-A to print "%" character as C standard, which
may be used in platform porting to print percentage information.
Signed-off-by: Heyi Guo <guoheyi@linux.alibaba.com>
Change-Id: I7af2f1d153548e426f423fce15dc48b0da56c622

lib/libc/printf.c

 /*
- * Copyright (c) 2014-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2014-2021, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -108,6 +108,9 @@ int vprintf(const char *fmt, va_list args)
 			/* Check the format specifier */
 loop:
 			switch (*fmt) {
+			case '%':
+				(void)putchar('%');
+				break;
 			case 'i': /* Fall through to next one */
 			case 'd':
 				num = get_num_va_args(args, l_count);