ARM: vexpress: fix CPU hotplug with CT9x4 tile.

The Cortex A9 tile fails to unplug CPUs if errata 643719 is not enabled.
This leads to random weird behaviours, but ultimately seem to lock the
kernel one way or another when a CPU is hot unplugged.

Symptoms range from a spinlock lockup in the scheduler, the entire
system hanging, to dumping out the kernel printk buffer a few lines at
a time, and other weird behaviours.

This is caused by the outgoing CPU not having its inner caches properly
flushed before it exits coherency - flush_cache_louis() is used to
achieve this, but as a result of the hardware bug, this function ends
up doing nothing without the errata workaround enabled.

As the Versatile Express has an affected CPU, this errata must always
be enabled.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>