Move GIC setup to a separate file

GIC setup code which used to be in bl31_plat_setup.c is now in fvp_gic.c
to simplify future changes to other bootloader stages.  This patch moves
code from bl31_plat_setup.c to fvp_gic.c, simplifies the include file
list for bl31_plat_setup.c, moves GIC declarations from the bl31.h header
file into the platform.h, and reworks files according to coding style
guide.

Change-Id: I48d82a4ba33e7114dcc88f9ca98767a06cf8f417