refactor(bl2): make post image handling platform-specific

In certain instances a platform may need to make modifications to an
image after it has been loaded by BL2. The existing common
implementation is a thin wrapper for a more generic arm post image
handler. To enable platforms to make changes to images when
they're loaded, move this into platform code.

Change-Id: I44025391056adb2d8a8eb4ea5984257b02027181
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/plat/arm/board/a5ds/a5ds_bl2_setup.c b/plat/arm/board/a5ds/a5ds_bl2_setup.c
index 1979c50..a0aa639 100644
--- a/plat/arm/board/a5ds/a5ds_bl2_setup.c
+++ b/plat/arm/board/a5ds/a5ds_bl2_setup.c
@@ -16,3 +16,8 @@
 {
 	arm_bl2_platform_setup();
 }
+
+int bl2_plat_handle_post_image_load(unsigned int image_id)
+{
+	return arm_bl2_plat_handle_post_image_load(image_id);
+}