arm: Create a GIC base class and make the PL390 derive from it

This patch moves the GIC interface to a separate base class and makes
all interrupt devices use that base class instead of a pointer to the
PL390 implementation. This allows us to have multiple GIC
implementations. Future implementations will allow in-kernel GIC
implementations when using hardware virtualization.

--HG--
rename : src/dev/arm/gic.cc => src/dev/arm/gic_pl390.cc
rename : src/dev/arm/gic.hh => src/dev/arm/gic_pl390.hh

src/dev/arm/SConscript

@@ -40,12 +40,14 @@
 Import('*')
 
 if env['TARGET_ISA'] == 'arm':
+    SimObject('Gic.py')
     SimObject('RealView.py')
 
     Source('a9scu.cc')
     Source('amba_device.cc')
     Source('amba_fake.cc')
-    Source('gic.cc')
+    Source('base_gic.cc')
+    Source('gic_pl390.cc')
     Source('pl011.cc')
     Source('pl111.cc')
     Source('kmi.cc')