x86: dts: Remove coreboot_fb.dtsi

x86: dts: Remove coreboot_fb.dtsi

There is no need to keep a separate coreboot_fb.dtsi since now we
have a generic coreboot payload dts.

While we are here, this also remove the out-of-date description in
the documentation regarding to coreboot framebuffer driver with
U-Boot loaded as a payload from coreboot. As the testing result with
QEMU 2.5.0 shows, the driver just works like a charm.

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