Documentation: Detail permitted DT properties for the BU21013 Touch Screen

Here we apply required documentation for the Rohm BU21013 Touch
Screen driver which describe available properties and how to use
them.

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: linux-input@vger.kernel.org
Cc: devicetree-discuss@lists.ozlabs.org
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>