arm/mach-vexpress: array accessed out of bounds

dcscb_allcpus_mask is an array of size 2.

The index variable cluster has to be checked against this limit
before accessing the array.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>