Looks like I spoke to soon. The AMMU comments were a bit confusing:
/* Peripheral regions: Large Page (512M); non-cacheable, posted */ /* config large page[1] to map 512MB VA 0x40000000 to L3 0x5FFFFFFF */
This page is only 512MB, which does not overlap the GPMC register address (0x50000000). I added a new large page and everything is working as expected.