xen/arm: keep track of reserved-memory regions

xen/arm: keep track of reserved-memory regions

As we parse the device tree in Xen, keep track of the reserved-memory
regions as they need special treatment (follow-up patches will make use
of the stored information.)

Reuse process_memory_node to add reserved-memory regions to the
bootinfo.reserved_mem array.

Refuse to continue once we reach the max number of reserved memory
regions to avoid accidentally mapping any portions of them into a VM.

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Acked-by: Julien Grall <julien.grall@arm.com>