Tidy up of the ft5406 driver to use DT (#2189)

Driver was using a fixed resolution, this commit
adds touchscreen size, and coordinate flip and swap
features via device tree overlays.

Adds overrides so the VC4 can adjust the DT parameters
appropriately; there is a newer version of the VC4 side
driver that can now set up the appropriate DT values
if required.

Signed-off-by: James Hughes <james.hughes@raspberrypi.org>