doc/examples/modem: Update modem.cfg with new features

Update the example config to contain IMSI and IMEI values which are used
when attaching to the network.

Set cpu-sched to realtime since RLCMAC is time-constrained.

Change-Id: I6060c833de2eef7591b7a4f149d9e148455db970

doc/examples/modem/modem.cfg

@@ -6,9 +6,19 @@ line vty
  no login
 !
 ms 1
+ imei 000000000000000 0
+ imei-fixed
+ sim test
+ test-sim
+  imsi 001010000000000
+  ki comp128 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+  no barred-access
+  rplmn 001 01
  apn internet
   tun-device modem4
 ! tun-netns netns_modem4
   type-support v4
   no shutdown
  no shutdown
+cpu-sched
+ policy rr 1