Good Resources:

• 
  EDK\doc\psf_rm.pdf : This file contains the formatting definitions for the MHS, MSS, MPD, PAO, BBD, MLD, MDD, and XBD file-types
• ​Designing a Custom Processor Peripheral Using Xilinx EDK :a great lengthy article on doing just as the title suggests, written by a Xilinx Engineer. Otherwise just search for “peripherals TechXclusives” on the Xilinx website.