ARM: vexpress: DCSCB: tighten CPU validity assertion

Currently the cpu argument validity check uses a hardcoded limit of 4.
The DCSCB configuration data provides the actual number of CPUs and
we already use it elsewhere.  Let's improve the cpu argument validity
check by using that information instead.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>