ARM: dts: ux500: Move Synaptics to right include

The Synaptics RMI4 touchscreen is a property of the
TVK1281618 R2 UIB, so move it into that file instead
of the main TVK1281618 main include so we can define
another touchscreen for the R3 UIB.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>