Scales much better than scanning the address range linearly.

v2: store pfn instead of address
Signed-off-by: NChristian König <christian.koenig@amd.com>
Tested-by: NMichel Dänzer <michel.daenzer@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>