sepolicy: allow rmt_storage to access sysfs_ssr context with read permission

This changes allow rmt_storage to read file
/sys/bus/msm_subsys/devices/subsys<>/name with sysfs_ssr context.
This is required by libmdmdetect to determine the presence of modem
subsystem at run time.

Change-Id: I29cb0c43cd4f428bfaa42fedb428afe759690fd0