selinux: Relocate selinux_enforcing to a separate 4k page

We need to relocate selinux_enforcing to a separate 4k page
to enable EL2 Hypervisor to monitor changes to this variable
using ARM stage 2 MMU. We will avoid getting page faults from
un-related data as the MMU granularity is configured to 4k.

Change-Id: I574f2b10af9fb46720bfbdd1a61eea7b841c2c39
Signed-off-by: Zhen Kong <zkong@codeaurora.org>