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
1 file changed