ARM: dts: qcom: msm8974-hammerhead: add touchscreen support

Add support for the Synaptics RMI4 touchscreen that is found on the
Nexus 5.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>