ARM: vexpress: use ARM_CPU_SUSPEND if needed

The vexpress tc2 power management code calls mcpm_loopback, which
is only available if ARM_CPU_SUSPEND is enabled, otherwise we
get a link error:

arch/arm/mach-vexpress/built-in.o: In function `tc2_pm_init':
arch/arm/mach-vexpress/tc2_pm.c:389: undefined reference to `mcpm_loopback'

This explicitly selects ARM_CPU_SUSPEND like other platforms that
need it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 3592d7e0 ("ARM: 8082/1: TC2: test the MCPM loopback during boot")
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>