nb/intel/fsp_sandybridge/gma: Set up OpRegion in nb code

Set up IGD OpRegion in northbridge and fill in GNVS' aslb.
At this point GNVS already has been set up by SSDT injection.

Required for future VBT patches that will:
* Use ACPI memory instead of CBMEM
* Use common implementation to locate VBT
* Fill in platform specific values

Change-Id: Ie5d93117ee8bd8d15085aedbfa7358dfcf5f0045
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/19307
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
3 files changed