ARM: 6960/1: allow enabling SCU code on UP

The scu_power_mode function can be used on UP builds as it drives signals
to an SOC power controller. So make it selectable for !SMP.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>