fs_mgr: enable DT fstab based on its status node appropriately

Follow the typical device tree guidelines that a node is only enabled
if it does not have a 'status' property or if it does have a 'status'
property with a value of 'ok' or 'okay'.

Bug: 117933812
Test: status='disabled' DT fstab is not present when reading fstab
Change-Id: Icb4cbe654757658460aabfdb952d71fb8354e09e