Makefile: add drivers/video/u_boot_logo.S to clean

make sandbox_defconfig
make mrproper
make tests

fails with

../drivers/video/u_boot_logo.S: Assembler messages:
../drivers/video/u_boot_logo.S:5: Error: file not found: drivers/video/u_boot_logo.bmp

We have to delete the generated file.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>