* Qualcomm SSBI

Some Qualcomm MSM devices contain a point-to-point serial bus used to
communicate with a limited range of devices (mostly power management
chips).

These require the following properties:

- compatible: "qcom,ssbi"

- qcom,controller-type
  indicates the SSBI bus variant the controller should use to talk
  with the slave device.  This should be one of "ssbi", "ssbi2", or
  "pmic-arbiter".  The type chosen is determined by the attached
  slave.

The slave device should be the single child node of the ssbi device
with a compatible field.