dtoverlays: Update 7inch DSI display overlay to use newer drivers

The older panel-raspberrypi-touchscreen driver had issues in
that it also controlled the power for the touchscreen without
having an appropriate hook for the touchscreen driver to control
that.

Mainline has now added a Toshiba TC358762 bridge driver, and
a regulator/backlight driver for the ATTiny microcontroller on
the board. That allows clean integration with the touchscreen
driver.

Switch the overlays over to using newer drivers.

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