Kconfig: simple panel requires backlight

During build of simple panel driver backlight is needed so let's enable
it:

aarch64-linux-gnu-ld.bfd: drivers/built-in.o: in function `simple_panel_set_backlight':
u-boot/drivers/video/simple_panel.c:43: undefined reference to `backlight_set_brightness'
aarch64-linux-gnu-ld.bfd: drivers/built-in.o: in function `simple_panel_enable_backlight':
u-boot/drivers/video/simple_panel.c:28: undefined reference to `backlight_enable'

Signed-off-by: Marcin Juszkiewicz <marcin@juszkiewicz.com.pl>