Export region_is_subregion() for determinig if a region is a child of a parent region. BUG=b:69614064 Change-Id: I6363fe4fdb33342f9d025a726ec7e17e87adf7e0 Signed-off-by: Aaron Durbn <adurbin@chromium.org> Reviewed-on: https://review.coreboot.org/22878 Tested-by: build bot (Jenkins) <no-reply@coreboot.org> Reviewed-by: Furquan Shaikh <furquan@google.com>