Makefile: remove generated font files

If `make mrproper` does not delete the generated drivers/video/fonts/*.S
files a following `make tests` fails.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>