Hi Radhika@adsetech,
All the information available on Quark is posted on this link https://communities.intel.com/community/makers/galileo/documentation/quarkdocuments . There is no specific document that discusses non-secure boot. You can still find helpful information on non-secure boot in the Quark documents. Some documents like https://communities.intel.com/docs/DOC-22477 have some useful information on the topic.
Regards
Sergio