arm_fpga: Add support to populate the CPU nodes in the DTB

arm_fpga: Add support to populate the CPU nodes in the DTB

At the moment BL31 dynamically discovers the CPU topology of an FPGA
system at runtime, but does not export it to the non-secure world.
Any BL33 user would typically looks at the devicetree to learn about
existing CPUs.

This patch exports a minimum /cpus node in a devicetree to satisfy
the binding. This means that no cpumaps or caches are described.
This could be added later if needed.

An existing /cpus node in the DT will make the code bail out with a
message.

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I589a2b3412411a3660134bdcef3a65e8200e1d7e