Add the pmbus driver for the Infineon ir38064 voltage regulator.

VOUT_MODE is not supported by the device.  The driver fakes linear16
mode with exponent value -8.

The device supports VOUT_PEAK, IOUT_PEAK, and TEMPERATURE_PEAK, however
this driver does not enable them.
Signed-off-by: Maxim Sloyko <maxims@google.com>
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>