of: spmi: Add support for device naming and lookups

Since we support multiple device_nodes per spmi_device when used
with the spmi-dev-container flag, it's often unclear how a driver
should make reference to a particular device. Therefore,
introduce the spmi specific binding spmi-dev-name that specifies
the device name for the device_node.

Also introduce an API that can be used from
the driver to lookup the specific device node associated with a
particular device name.

Note that it may seem at first glance that the binding 'label' is
redundant with device_node->name. However, per the ePAPR,
device_node->name is intended to be as generic as possible,
representing the function of the device and not the precise
programming model. 'label' is used to give a platform
specific name that can be queried from device drivers.

Change-Id: I28eca35d0b625372c26a6d8ad81e7679f200d14b
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>