mfd: PM8018: Add GPIO digital voltage level definitions

PM8018 GPIOs need default voltage level to initialize.
The voltage level is used during the defalut configuration
setup during boot up and at any time when gpio needs to be
reconfigured.

Add these voltage level definitions so that they can be used
for such PM8018 GPIO voltage level configuration.

Change-Id: Ifc5e77e976826e3493a972f9b0f6a412709acb33
Signed-off-by: Jay Chokshi <jchokshi@codeaurora.org>