Simplify spin logic

The logic for initial boot is more complicated than it needs to be,
with both first_spin() having a special case for CPU0 that requires an
additional argument to be passed to spin().

Simplify this by moving the special-case logic for CPU0 into
first_spin(). This removes the need to provide a dummy mailbox for CPU0
to spin on, simplfiies callers of first_spin() and spin(), which no
longer need to pass a dummy mailbox or 'is_entry' for CPU0.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Cc: Akos Denke <akos.denke@arm.com>
Cc: Luca Fancellu <luca.fancellu@arm.com>