dt-bindings: mfd: da9052: Support TSI as ADC

DA9052 allows using the touchscreen input pins as general purpose
analogue input pin by wiring analogue inputs to X+, X-, Y+ and Y-
and providing a reference voltage at TSIREF pin.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>