dt-bindings: qpnp-amoled: add support to configure single phase for IBB

On certain PMICs like PM8350B, IBB can be configured in single or
dual or auto phase based on the panel configuration. Though dual
or auto phase is mostly used, forced single phase configuration
is needed for some hardware. Add support for it.

Change-Id: I49206d1a68392362a32fd4a193f0891ccae3078d