arm: Add a GICv2m device

This patch adds a new PIO-accessible GICv2m shim. This shim has a PIO
slave port on one side, and SPI 'wires' on the other. It accepts MSIs
from the system and triggers SPIs on the GIC. It is configurable with
a number of frames, each of which has a number of SPIs and a base SPI
offset.

A Linux driver for GICv2m is available upstream.

src/dev/arm/SConscript

@@ -50,6 +50,7 @@ if env['TARGET_ISA'] == 'arm':
     Source('base_gic.cc')
     Source('generic_timer.cc')
     Source('gic_pl390.cc')
+    Source('gic_v2m.cc')
     Source('pl011.cc')
     Source('pl111.cc')
     Source('hdlcd.cc')
@@ -65,6 +66,7 @@ if env['TARGET_ISA'] == 'arm':
     DebugFlag('AMBA')
     DebugFlag('HDLcd')
     DebugFlag('PL111')
+    DebugFlag('GICV2M')
     DebugFlag('Pl050')
     DebugFlag('GIC')
     DebugFlag('RVCTRL')