The ARM_GIC_ARCH build option was supposed to default to 2 on all
platforms. However, the default value was set in the FVP makefile
so for all other platforms it wasn't even defined.
This patch moves the default value to the main Makefile. The platform
port can then override it if needed.
Change-Id: I8e2da1cce7ffa3ed18814bbdcbcf2578101f18a6