dtoverlays: Use edt-ft5506 for 10 points, instead of edt-ft5x06

Whilst all the datasheets describe FT5x06 as supporting "up to
10 points of absolution X and Y coordinates", the driver
implementation for the compatible string "edt,edt-ft5x06" only
allows for 5.

Switch to the "edt,edt-ft5506" compatible string which allows for
10 points with no other differences.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>