mtx: clean up caching of page state

map_restore restored the map but not the cache of the write only port, this
then caused crashes if interrupt returns occurred during a user memory
access.

Use the helpers throughout for neatness
mtx: README

Explain the shortcut way to load the images
mtx: Clean up the port and tidy loose ends

Memory sizing
DART setup
Better CTC configuration
Use IM2 so we can tell CTC and DART interrupts apart