system_server: Adds permission to system_server to write sysfs file

Need this for changing the max_cpufreq and min_cpufreq for the low power
mode.

Denials:
type=1400 audit(1402431554.756:14): avc: denied { write } for pid=854
comm="PowerManagerSer" name="scaling_max_freq" dev="sysfs" ino=9175
scontext=u:r:system_server:s0 tcontext=u:object_r:sysfs_devices_system_cpu:s0
tclass=file

Change required for Change-Id: I1cf458c4f128818ad1286e5a90b0d359b6913bb8

Change-Id: Ic5ce3c8327e973bfa1d53f298c07dcea1550b646
Signed-off-by: Ruchi <Kandoi&lt;kandoiruchi@google.com>

system_server.te

@@ -143,6 +143,7 @@ selinux_check_access(system_server)
 # XXX Label sysfs files with a specific type?
 allow system_server sysfs:file rw_file_perms;
 allow system_server sysfs_nfc_power_writable:file rw_file_perms;
+allow system_server sysfs_devices_system_cpu:file w_file_perms;
 
 # Access devices.
 allow system_server device:dir r_dir_perms;