tiny-printf: Support sprintf()

Add a simple version of this function for SPL. It does not check the buffer
size as this would add to the code size.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefan Roese <sr@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: lesne@alse-fr.com
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Sylvain Lesne <lesne@alse-fr.com>
Tested-by: Sylvain Lesne <lesne@alse-fr.com>