dt-bindings: arm: bcm: raspberrypi,bcm2835-firmware: Add touchscreen child node

Convert Raspberry Pi firmware 7" touchscreen controller device tree
binding to json-schema.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250925153144.4082786-2-dario.binacchi@amarulasolutions.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>