drivers/intel/gma: Hook up libgfxinit

Add `libgfxinit` as another option for native graphics initialization.
For that, the function gma_gfxinit() (see drivers/intel/gma/i915.h) has
to be called by the respective northbridge/soc code.

A mainboard port needs to select `CONFIG_MAINBOARD_HAS_LIBGFXINIT` and
implement the Ada package `GMA.Mainboard` with a single function `ports`
that returns a list of ports to be probed for displays.

v2: Update 3rdparty/libgfxinit to its latest master commit to make
    things buildable within coreboot.

v3: Another update to 3rdparty/libgfxinit. Including support to select
    the I2C port for VGA.

Change-Id: I4c7be3745f32853797d3f3689396dde07d4ca950
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/16952
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/drivers/intel/gma/i915.h

@@ -309,4 +309,7 @@ void
 generate_fake_intel_oprom(const struct i915_gpu_controller_info *conf,
 			  struct device *dev, const char *idstr);
 
+/* interface to libgfxinit (gma.adb) */
+void gma_gfxinit(u64 mmio_base, u64 linear_fb, u32 phys_fb, int *success);
+
 #endif