configs/infra: introduce the definition ARM_TF_RAS_FW_FIRST

Introduce a flag 'ARM_TF_RAS_FW_FIRST' that can then be used to enable
the required poritions of the firmware stack to support firmware first
error handling.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: Iaf6a2e2f742b1e78ed8c8b1383d836d72f1f566b