soc/intel/skylake: Update GFX devtree options

soc/intel/skylake: Update GFX devtree options

This patch includes the following changes:

 1. Sets FSP options in romstage_fsp20.c to select primary GPU.
    List of options:
      - InternalGfx,
      - PrimaryDisplay.

 2. iGPU will be initialized if the corresponding PCI device is defined
    in the device tree as:

      device pci 02.0 on end

    In this case, it is not necessary to set the InternalGfx option to
    enable this device

 3. Primary_iGFX is used as the default value for all skl/kbl boards
    (since the PrimaryDisplay option isn`t defined in the devicetree.cb)

Change-Id: Ie3f9362676105e41c69139a094dbb9e8b865689f
Signed-off-by: Maxim Polyakov <max.senia.poliak@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32044
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>