RealView: Move the SCU initialisation out of __v6_setup

This patch moves the SCU initialisation from __v6_setup to the
smp_prepare_cpus() function as it relies on platform-specific
settings. Changes to get_core_count() are mainly for allowing cleaner
code with the upcoming PB11MPCore patches.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>