layout: Implement flashprog_layout_get_region_range()

Similar to the function found in flashrom/master, implement a
libflashprog API to query the range of a layout region.

Change-Id: Idb3b1a4d24a34847102d66ee0e2cb4d93ae99eac
Signed-off-by: Nico Huber <nico.h@gmx.de>
diff --git a/libflashprog.map b/libflashprog.map
index 9bf8ec0..a0dd125 100644
--- a/libflashprog.map
+++ b/libflashprog.map
@@ -11,6 +11,7 @@
     flashprog_image_write;
     flashprog_init;
     flashprog_layout_add_region;
+    flashprog_layout_get_region_range;
     flashprog_layout_include_region;
     flashprog_layout_new;
     flashprog_layout_read_fmap_from_buffer;