dev-arm: Remove the dependency of Platform for ArmSigInterruptPin (#878)

ArmSigInterruptPin don't send the interrupt to GIC. Instead it sends the
interrupt to the irq specified in Param. When using ArmSigInterruptPin,
we shouldn't ask users to provide "Platform" since it doesn't need it.
To reduce the confusion, this change removes the dependency of Platform
for ArmSigInterruptPin.

Change-Id: I0ee507ed1c08b4fa6d3e384e28732f3acb4f6892

src/dev/arm/Gic.py

@@ -155,6 +155,7 @@ class ArmSigInterruptPin(ArmInterruptPin):
     cxx_class = "gem5::ArmSigInterruptPinGen"
 
     irq = IntSourcePin("Interrupt pin")
+    platform = NULL
 
 
 class GicV2(BaseGic):

src/dev/arm/base_gic.cc

@@ -154,7 +154,6 @@ ArmInterruptPin::ArmInterruptPin(
       : threadContext(tc), platform(dynamic_cast<RealView*>(p.platform)),
         intNum(p.num), triggerType(p.int_type), _active(false)
 {
-    fatal_if(!platform, "Interrupt not connected to a RealView platform");
 }
 
 void
@@ -190,6 +189,7 @@ ArmSPI::ArmSPI(
     const ArmSPIParams &p)
       : ArmInterruptPin(p, nullptr)
 {
+    fatal_if(!platform, "Interrupt not connected to a RealView platform");
 }
 
 void
@@ -210,6 +210,7 @@ ArmPPI::ArmPPI(
     const ArmPPIParams &p, ThreadContext *tc)
       : ArmInterruptPin(p, tc)
 {
+    fatal_if(!platform, "Interrupt not connected to a RealView platform");
 }
 
 void