Input: add official Raspberry Pi's touchscreen driver

Add's support to Raspberry Pi's 7" Touch device. Instead of using a
conventional bus all information is copied into a memory mapped area by
RPi's firmware.

Based on the driver found in RPi's kernel repository.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>