ARM: dts: msm8974-FP2: Introduce gpio-keys nodes

This introduces the gpio-keys nodes for keys of the FP2 and the
associated pinctrl state.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>