Add needed code to pddf_custom_psu.c to deal with multi PSU and get SN.
How to verify it
Plugin new PSU (3Y) and test,
```
root@sonic:/sys/bus/i2c/drivers/psu/9-0050# cat psu_serial_num
S0A000X601919000013
root@sonic:/sys/bus/i2c/drivers/psu/9-0050# cat psu_model_name
YESM1300AM
root@sonic:/home/admin# pddf_psuutil mfrinfo
PSU Status Manufacturer ID Model Serial Fan Airflow Direction
PSU1 NOT OK 3Y POWER YESM1300AM S0A000X601919000007 exhaust
PSU2 OK 3Y POWER YESM1300AM S0A000X601919000013 exhaust
```
Co-authored-by: Jostar Yang <jostar_yang@accton.com.tw>
