drv/intel/gma/opregion: Add interface for GNVS ASLB handling

Add and use new interface to set and get GNVS' ASLB register.
To be used by Intel's gma driver to set ASLB at ACPI table
creation and to get ASLB on S3 resume.

Change-Id: If30c6b2270069783b0892774802f47406404da5f
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/20435
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/drivers/intel/gma/opregion.h

@@ -246,5 +246,7 @@ typedef struct {
 } __attribute__((packed)) optionrom_vbt_t;
 
 void intel_gma_opregion_register(uintptr_t opregion);
+uintptr_t gma_get_gnvs_aslb(const void *gnvs);
+void gma_set_gnvs_aslb(void *gnvs, uintptr_t aslb);
 
 #endif /* _COMMON_GMA_H_ */