Update pathnames in maintainers.rst file

The maintainers.rst file lists files and directories that each contributor looks
after in the TF-A source tree. As files and directories move around over time,
some pathnames had become invalid. Fix them, either by updating the path if
it has just moved, or deleting it altogether if it doesn't seem to exist
anymore.

Change-Id: Idb6ff4d8d0b593138d4f555ec206abcf68b0064f
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>