arm_fpga: Add support for unknown MPIDs

This patch allows the system to fallback to a default CPU library
in case the MPID does not match with any of the supported ones.

This feature can be enabled by setting SUPPORT_UNKNOWN_MPID build
option to 1 (enabled by default only on arm_fpga platform).

This feature can be very dangerous on a production image and
therefore it MUST be disabled for Release images.

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I0df7ef2b012d7d60a4fd5de44dea1fbbb46881ba