input: qpnp-power-on: add device tree constants for power on/off types

Add PMIC PON peripheral power-on and power-off types in a device
tree header file.  Use these constants in the qpnp-power-on
driver enums to ensure that device tree and the driver are always
in-sync.  Also update device tree binding documentation to match
what is supported by the driver.

Change-Id: I77f924f846380a3eaf968de6c85a8680ed0ca6ea
Signed-off-by: David Collins <collinsd@codeaurora.org>