device-tree: nexus7: Remove power gpio key entry and use pmic8xxx-pwrkey

Since the pmic8xxx-pwrkey driver is already supported in the
qcom-apq8064.dtsi, and the pmic8xxx-pwrkey supports logic to
configure proper device shutdown when ps_hold goes low, it is
better to use that driver then a generic gpio button.

Thus this patch remove the gpio power key entry here, so we
don't get double input events from having two drivers enabled.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Andy Gross <agross@codeaurora.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: linux-arm-msm@vger.kernel.org
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>