power: vexpress: make dev_attr_active static

The dev_attr_active is not exported or defined to be used
outside the driver, so make it static to avoid the following
warning:

drivers/power/reset/vexpress-poweroff.c:77:1: warning: symbol 'dev_attr_active' was not declared. Should it be static?

Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>