mach-snapdragon: switch to PMIC button driver

The PMIC button driver is a much better representation of the hardware
here, adjust the boards to use upstream DT and the PMIC button driver
instead of exposing the buttons as GPIOs and relying on the GPIO-button
driver.

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>