fwk: Add new function to retrieve sub-element id
Created by: tarek-arm
It is required to convert from sub-element index to identifier. This patch adds the required new function.
Signed-off-by: Tarek El-Sherbiny tarek.el-sherbiny@arm.com Change-Id: I55f2702668ef97f7a6964adb924c3cedaae1e1e1