kernel: Restrict permissions of /proc/iomem.

The permissions of /proc/iomem currently are -r--r--r--. Everyone can
see its content. As iomem contains information about the physical memory
content of the device, restrict the information only to root.

Change-Id: If0be35c3fac5274151bea87b738a48e6ec0ae891
bug: 28814213
CRs-Fixed: 786116
Signed-off-by: Biswajit Paul <biswajitpaul@codeaurora.org>
Signed-off-by: Avijit Kanti Das <avijitnsec@codeaurora.org>