boot/Kconfig: Add Video Kconfig as dependency for FDT_SIMPLEFB

boot/Kconfig: Add Video Kconfig as dependency for FDT_SIMPLEFB

The fdt_simplefb.c APIs rely on video-uclass APIs and structures to
fill/update framebuffer information, so compile it only when VIDEO
Kconfig is enabled, as otherwise below warning can be seen if VIDEO
Kconfig is disabled:

"boot/fdt_simplefb.c:96:12: warning: fdt_simplefb_enable_existing_node
defined but not used [-Wunused-function]
96 | static int fdt_simplefb_enable_existing_node(void *blob)"

Reported-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Devarsh Thakkar <devarsht@ti.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>