defconfig: sdm855: enable PMIC power-on driver

Enable the PMIC power-on driver so that software can receive
power key and volume down key presses and also configure the
PMICs appropriately for different power-off types.

Change-Id: Ib47f972c34b37b74aed9837aad101af00f499cf4
Signed-off-by: David Collins <collinsd@codeaurora.org>