[pddf]: Adding pre_pddf_init script in the service file (#7834)

Why I did it
Some platforms need to run few steps before the PDDF service is actually started.

* Adding pre_pddf_init script in the service file
* Raising exception for get_target_speed() for PSU-fan in PDDF (#8129)

platform/pddf/i2c/service/pddf-platform-init.service

@@ -5,6 +5,7 @@ DefaultDependencies=no
 
 [Service]
 Type=oneshot
+ExecStartPre=-/usr/local/bin/pre_pddf_init.sh
 ExecStart=/usr/local/bin/pddf_util.py install
 ExecStop=/usr/local/bin/pddf_util.py clean
 RemainAfterExit=yes

platform/pddf/platform-api-pddf-base/sonic_platform_pddf_base/pddf_fan.py

@@ -224,7 +224,7 @@ class PddfFan(FanBase):
         target_speed = 0
         if self.is_psu_fan:
             # Target speed not usually supported for PSU fans
-            target_speed = 0
+            raise NotImplementedError
         else:
             idx = (self.fantray_index-1)*self.platform['num_fans_pertray'] + self.fan_index
             attr = "fan" + str(idx) + "_pwm"