plat(common): move to platform-independent setup code

Upcoming RD-Fremont platforms have identical platform setup code to fvp.
Move the plat_setup file to plat/common directory structure, while
preserving platform directories in the case there is a platform with
different setup routines.
The platform directory now contains the CMake file, any actual platform
specific code, and the plat_private header.

Signed-off-by: Harry Moulton <harry.moulton@arm.com>
Change-Id: Ieaa1ebd6aa1d8ad85402a577bacd6b4c9ed72cf8