drivers/intel/gma: Hook up libgfxinit in romstage

drivers/intel/gma: Hook up libgfxinit in romstage

A mainboard port needs to:

- select `CONFIG_MAINBOARD_HAS_EARLY_LIBGFXINIT'

- implement the Ada package `GMA.Mainboard' with a single function
  `ports' that returns a list of ports to be probed for displays.

- set the desired `GFX_GMA_DEFAULT_MMIO' IO memory address to use
  in romstage (and ramstage) for the graphic device.

BUG=b:252792591
BRANCH=firmware-brya-14505.B
TEST=libgfxinit compiles in romstage.
     libgfxinit successfully executes in romstage and ramstage using
     the requested MMIO setting on skolas.

Change-Id: I3c2101de10dc5df54fe873e43bbe0f1c4dccff44
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70276
Reviewed-by: Tarun Tuli <taruntuli@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>