commonlib/region: allow empty mmap()/munmap() in region_device_ops

Instead of assuming all region_devices have an mmap() and munmap()
implementation fail those calls when one isn't provided.

BUG=chrome-os-partner:56151

Change-Id: I9b03e084aa604d52d6b5bab47c0bf99d9fbcd422
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16190
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/commonlib/region.c

@@ -74,6 +74,9 @@ void *rdev_mmap(const struct region_device *rd, size_t offset, size_t size)
 
 	rdev = rdev_root(rd);
 
+	if (rdev->ops->mmap == NULL)
+		return NULL;
+
 	return rdev->ops->mmap(rdev, req.offset, req.size);
 }
 
@@ -83,6 +86,9 @@ int rdev_munmap(const struct region_device *rd, void *mapping)
 
 	rdev = rdev_root(rd);
 
+	if (rdev->ops->munmap == NULL)
+		return -1;
+
 	return rdev->ops->munmap(rdev, mapping);
 }