sepolicy : add selable for "proc/device-tree/cpus"

CTS test testDeviceTreeCpuCurrent expect "proc/device-tree/cpus"
should be readable where CTS is running as untrusted app and by
default this is going to get denied for read  to /proc.
Setting the label to sysfs_devices_system_cpu which is global
read.

Change-Id: Idfd6f7cce00a28359d914e9b56d35ea2f4b8bb9f
avc: denied { read } for name="cpus" dev="sysfs" ino=152 scontext=u:r:untrusted_app:s0:c512,c768
           tcontext=u:object_r:sysfs:s0 tclass=dir permissive=0
CRs-Fixed: 1065129