Add the pmbus driver for the STMicroelectronics pm6764 voltage regulator.

the output voltage use the MFR_READ_VOUT 0xD4
vout value returned is linear11
Signed-off-by: Charles Hsu <hsu.yungteng@gmail.com>
[groeck: Fixed various compile errors; marked pm6764tr_of_match __maybe_unused]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>