Improvements to ARM GIC driver

This patch introduces several improvements to the ARM GIC driver:

* In function gicd_set_itargetsr(), target CPU is specified using
  the same bit mask detailed in the GICD_ITARGETSRn register instead
  of the CPU linear ID, removing the dependency between bit position
  and linear ID in the platform porting. The current CPU bit mask
  may be obtained by reading GICD_ITARGETSR0.

* PPIs and SGIs are initialized in arm_gic_pcpu_distif_setup().
  SPIs are initialized in arm_gic_distif_setup().

* By default, non secure interrupts are assigned the maximum
  priority allowed to a non secure interrupt (defined by
  GIC_HIGHEST_NS_PRIORITY).

* GICR base address is allowed to be NULL for GICv1 and GICv2.

Change-Id: Ie2837fe860d43b2282e582dfdb13c39c6186f232
diff --git a/include/drivers/arm/gic_v2.h b/include/drivers/arm/gic_v2.h
index 4c6b0dc..a2d3eee 100644
--- a/include/drivers/arm/gic_v2.h
+++ b/include/drivers/arm/gic_v2.h
@@ -48,6 +48,7 @@
 #define GIC_HIGHEST_NS_PRIORITY	128
 #define GIC_LOWEST_NS_PRIORITY	254 /* 255 would disable an interrupt */
 #define GIC_SPURIOUS_INTERRUPT	1023
+#define GIC_TARGET_CPU_MASK	0xff
 
 #define ENABLE_GRP0		(1 << 0)
 #define ENABLE_GRP1		(1 << 1)