Physical memory section mappings should not require 64k alignment

This causes a flaky test in kmtest:MmSection (since if we happen to have 64k alignment it will succeed in ROS, otherwise it fails).
E.g.
https://reactos.org/testman/diff.php?id1=31936068&id2=31940842&type=1&strip=0
https://reactos.org/testman/compare.php?ids=52432,52438,52441,52442