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

This introduces the gpio-keys node for keys of hammerhead and pinctrl
state associated with it.

Cc: Andy Gross <andy.gross@linaro.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: David Brown <david.brown@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-arm-msm@vger.kernel.org
Cc: linux-soc@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Bhushan Shah <bshah@kde.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>