dt-bindings: Document the Raspberry Pi Touchscreen nodes.

This doesn't yet cover input, but the driver does get the display
working when the firmware is disabled from talking to our I2C lines.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170927193654.12609-3-eric@anholt.net