How to write an EDAC memory controller driver

Actually surprisingly easy.

You need some experience of C, and a familiarity with binary logic and arithmetic.

Buy, or read on line Linux Device Drivers, 3rd Edition - http://lwn.net/Kernel/LDD3/ - read the introduction, and the section on PCI devices at a minimum.
Download memory controller data sheet (e.g. from Intel, or AMD).
Read up on memory configuration, and ECC reporting sections of data sheet.
Find an existing driver for a similarly structured chip and modify it.