hw/i2c: pmbus: add vout mode bitfields

The VOUT_MODE command is described in the PMBus Specification,
Part II, Ver 1.3 Section 8.3

VOUT_MODE has a three bit mode and 4 bit parameter, the three bit
mode determines whether voltages are formatted as uint16, uint16,
VID, and Direct modes. VID and Direct modes use the remaining 5 bits
to scale the voltage readings.

Reviewed-by: Hao Wu <wuhaotsh@google.com>
Acked-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Titus Rwantare <titusr@google.com>
Message-ID: <20231023-staging-pmbus-v3-v4-2-07a8cb7cd20a@google.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

include/hw/i2c/pmbus_device.h

@@ -444,6 +444,14 @@ typedef struct PMBusCoefficients {
     int32_t R;     /* exponent */
 } PMBusCoefficients;
 
+/**
+ * VOUT_Mode bit fields
+ */
+typedef struct PMBusVoutMode {
+    uint8_t  mode:3;
+    int8_t   exp:5;
+} PMBusVoutMode;
+
 /**
  * Convert sensor values to direct mode format
  *