Update the tests_mark_conditions_platform_tests (#7547)

What is the motivation for this PR?
The track Issue : #7546

How did you do it?
Add the check for asic_subtype for broadcom_dnx

How did you verify/test it?
Made sure the test_ser.py is skipped in DNX platforms

Author: judyjoseph

tests/common/plugins/conditional_mark/tests_mark_conditions_platform_tests.yaml

@@ -677,6 +677,7 @@ platform_tests/broadcom/test_ser.py:
     reason: "platform does not support test_ser"
     conditions:
       - "platform in ['x86_64-arista_720dt_48s'] and https://github.com/sonic-net/sonic-mgmt/issues/7297"
+      - "asic_subtype in ['broadcom-dnx'] and https://github.com/sonic-net/sonic-mgmt/issues/7546"
 
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 #####  cli/test_show_platform.py  #####