SecurityPkg: Fix SecureBootDefaultKeysDxe failed to start

SecurityPkg: Fix SecureBootDefaultKeysDxe failed to start

The dbt and dbx keys are optional, the driver entry should return
EFI_SUCCESS to start if they are not found in the firmware flash. This
patch is to fix it and update the description of retval as well.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Grzegorz Bernacki <gjb@semihalf.com>
Signed-off-by: Nhi Pham <nhi@os.amperecomputing.com>
Reviewed-by: Grzegorz Bernacki <gjb@semihalf.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>