gpio: qcom_pmic: drop pon GPIO driver

Remove the (now unused) GPIO driver for the power and resin buttons on
the PMIC.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>