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>

src/arch/x86/car.ld

@@ -72,6 +72,13 @@
 	 * runtime. Therefore, they can be placed in the _bss region. */
 #if CONFIG(ROMSTAGE_LIBHWBASE)
 	*(.data.hw__*_E)
+#endif
+#if CONFIG(EARLY_GFX_GMA)
+	*(.data.gma*_E)
+	/* libgfxinit uses a boolean variable to track its initialization
+	 * state. Since the initial value is False it can safely be placed in
+	 * the _bss region. */
+	*(.data.hw__gfx__gma__initialized)
 #endif
 	. = ALIGN(ARCH_POINTER_ALIGN_SIZE);
 	_ebss = .;