uboot-imx

x86: doc: Document coreboot framebuffer driver issue on QEMU

For some unknown reason, coreboot framebuffer driver never works on
QEMU since day 1. It seems the driver only works on real hardware.
Document this issue.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>