commonlib: Add function to compute relative offsets from two region_devices.

Provide a helper function which returns the relative offset
between 2 region_devices that have a parent-child child relationship.

BUG=chrome-os-partner:48412
BUG=chromium:445938
BRANCH=None
TEST=Utilized and booted on glados.

Change-Id: Ie0041b33e73a6601748f1289e98b6f1f8756eb11
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/12782
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/commonlib/region.c

@@ -48,6 +48,18 @@ static const struct region_device *rdev_root(const struct region_device *rdev)
 	return rdev->root;
 }
 
+ssize_t rdev_relative_offset(const struct region_device *p,
+				const struct region_device *c)
+{
+	if (rdev_root(p) != rdev_root(c))
+		return -1;
+
+	if (!is_subregion(&p->region, &c->region))
+		return -1;
+
+	return region_device_offset(c) - region_device_offset(p);
+}
+
 void *rdev_mmap(const struct region_device *rd, size_t offset, size_t size)
 {
 	const struct region_device *rdev;