Define ARM_GIC_ARCH default value for all platforms

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
Merge pull request #168 from sandrine-bailleux/sb/set-default-gic-arch

Define ARM_GIC_ARCH default value for all platforms