Make SCP_BL2(U) image loading configurable on CSS platforms

Make SCP_BL2(U) image loading configurable on CSS platforms

Current code mandates loading of SCP_BL2/SCP_BL2U images for all
CSS platforms. On future ARM CSS platforms, the Application
Processor (AP) might not need to load these images. So, these
items can be removed from the FIP on those platforms.

BL2 tries to load  SCP_BL2/SCP_BL2U images if their base
addresses are defined causing boot error if the images are not
found in FIP.

This change adds a make flag `CSS_LOAD_SCP_IMAGES` which if set
to `1` does:
1. Adds SCP_BL2, SCP_BL2U images to FIP.
2. Defines the base addresses of these images so that AP loads
   them.

And vice-versa if it is set to `0`. The default value is set to
`1`.

Change-Id: I5abfe22d5dc1e9d80d7809acefc87b42a462204a