In the multi console driver, allowing to register the same console more
than once may result in an infinte loop when putc is called.

If, for example, a boot message is trying to be printed, but the
consoles in the loop in the linked list are runtime consoles, putc will
iterate forever looking for a console that can print boot messages (or
a NULL pointer that will never come).

This loop in the linked list can occur after restoring the system from a
system suspend. The boot console is registered during the cold boot in
BL31, but the runtime console is registered even in the warm boot path.
Consoles are always added to the start of the linked list when they are
registered, so this it what should happen if they were actually
different structures:

   console_list -> NULL
   console_list -> BOOT -> NULL
   console_list -> RUNTIME -> BOOT -> NULL
   console_list -> RUNTIME -> RUNTIME -> BOOT -> NULL

In practice, the two runtime consoles are the same...