ARM: 6472/1: vexpress ct-ca9x4: only set twd_base if local timers are being used

In commit bde28b84, I made the assumption that CONFIG_SMP is always set
for the quad-core ct-ca9x4 platform. As it turns out, people who aren't
using the SMP goodness are confronted with a build failure.

This patch fixes this issue by ensure that twd_base is only set if
local timers are being used (and therefore SMP support is configured).

Reported-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>