GICv3: Fix the GICD_IROUTER offset

This patch fixes the offset of GICD_IROUTER register defined in gicv3.h.
Although the GICv3 documention mentions that the offset for this register
is 0x6100-0x7FD8, the offset calculation for an interrupt id `n` is :

   0x6000 + 8n, where n >= 32

This requires the offset for GICD_IROUTER to be defined as 0x6000.

Fixes ARM-software/tf-issues#410

Change-Id: If9e91e30d946afe7f1f60fea4f065c7567093fa8
diff --git a/include/drivers/arm/gicv3.h b/include/drivers/arm/gicv3.h
index e915c07..b7ad778 100644
--- a/include/drivers/arm/gicv3.h
+++ b/include/drivers/arm/gicv3.h
@@ -55,7 +55,11 @@
 #define GICD_SETSPI_SR		0x50
 #define GICD_CLRSPI_SR		0x50
 #define GICD_IGRPMODR		0xd00
-#define GICD_IROUTER		0x6100
+/*
+ * GICD_IROUTER<n> register is at 0x6000 + 8n, where n is the interrupt id and
+ * n >= 32, making the effective offset as 0x6100.
+ */
+#define GICD_IROUTER		0x6000
 #define GICD_PIDR2_GICV3	0xffe8
 
 #define IGRPMODR_SHIFT		5