Make test_reboot.py more stable on Cisco platform (sonic-net#12502)

Description of PR
Summary:
Fixes test_reboot.py::test_continous_reboot more stable on Cisco platform

Approach
What is the motivation for this PR?
The testcase is flakey on Cisco platform because Cisco router reboot time > 300s

How did you do it?
Increase wait_for_startup timeout value from 300 to 600

How did you verify/test it?
Run with Cisco 8111 router.

Any platform specific information?
No impact to other platform.

co-authorized by: [email protected]

tests/platform_tests/test_reboot.py

@@ -91,7 +91,7 @@ def reboot_and_check(localhost, dut, interfaces, xcvr_skip_list,
     check_interfaces_and_services(dut, interfaces, xcvr_skip_list, reboot_type=reboot_type)
     if dut.is_supervisor_node():
         for lc in duthosts.frontend_nodes:
-            wait_for_startup(lc, localhost, delay=10, timeout=300)
+            wait_for_startup(lc, localhost, delay=10, timeout=600)
             check_interfaces_and_services(lc, interfaces, xcvr_skip_list)