OPAL_SENSOR_READ
================

The OPAL sensor call reads a sensor data using a unique handler to
identity the targeted sensor. The `sensor_handler` is provided
via the device tree and is opaque to the OS (although we currently
do use an encoding scheme).

This call can be asynchronous, when a message needs to be sent to a
service processor for example.  In this case, the call will return
OPAL_ASYNC_COMPLETION and the token parameter will be used to wait for
the completion of the request.


Parameters
----------
::

	uint32_t sensor_handler
	int	 token
	uint32_t *sensor_data


Return values
-------------
OPAL_SUCCESS
  Success!

OPAL_PARAMETER
  invalid sensor handler

OPAL_UNSUPPORTED
  platform does not support reading sensors.

In case of communication with the FSP on IBM systems:

OPAL_ASYNC_COMPLETION
  a request was sent and an async completion will
  be triggered with the @token argument

OPAL_PARTIAL
  the request completed but the data returned is invalid

OPAL_BUSY_EVENT
  a previous request is still pending

OPAL_NO_MEM
  allocation failed

OPAL_INTERNAL_ERROR
  communication failure with the FSP

OPAL_HARDWARE
  FSP is not available
