Input: add touchscreen support for TS-4800

On this board, the touchscreen, an ads7843, is not handled directly by
Linux but by a companion FPGA. This FPGA is memory-mapped and the IP
design is very similar to the mk712.

This commit adds the support for this IP.

Signed-off-by: Damien Riegel <damien.riegel@savoirfairelinux.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>