KConfig: Vexpress: build the ARM_GLOBAL_TIMER with vexpress platform

The Cortex A5 now contains bindings for the global timer
driver. Ensure that the driver is built with a vexpress platform.

Signed-off-by: Matthew Leach <matthew.leach@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>