juno: Implement plat_report_exception() function

As for FVP platforms, Juno provides some LEDs that we can
use to report exceptions during the early boot code.

plat/juno/aarch64/plat_helpers.S

@@ -39,12 +39,21 @@
 	 * void plat_report_exception(unsigned int type)
 	 * Function to report an unhandled exception
 	 * with platform-specific means.
-	 * On FVP platform, it updates the LEDs
+	 * On Juno platform, it updates the LEDs
 	 * to indicate where we are
 	 * ---------------------------------------------
 	 */
 plat_report_exception:
-	/* Juno todo: Come up with a way of reporting errors */
+	mrs	x1, CurrentEl
+	lsr	x1, x1, #MODE_EL_SHIFT
+	lsl	x1, x1, #SYS_LED_EL_SHIFT
+	lsl	x0, x0, #SYS_LED_EC_SHIFT
+	mov	x2, #(SECURE << SYS_LED_SS_SHIFT)
+	orr	x0, x0, x2
+	orr	x0, x0, x1
+	mov	x1, #VE_SYSREGS_BASE
+	add	x1, x1, #V2M_SYS_LED
+	str	w0, [x1]
 	ret
 
 	/*

plat/juno/platform.h

@@ -148,6 +148,25 @@
 #define CNTACR_RWVT_SHIFT	0x4
 #define CNTACR_RWPT_SHIFT	0x5
 
+/* V2M motherboard system registers & offsets */
+#define VE_SYSREGS_BASE		0x1c010000
+#define V2M_SYS_LED		0x8
+
+/*
+ * V2M sysled bit definitions. The values written to this
+ * register are defined in arch.h & runtime_svc.h. Only
+ * used by the primary cpu to diagnose any cold boot issues.
+ *
+ * SYS_LED[0]   - Security state (S=0/NS=1)
+ * SYS_LED[2:1] - Exception Level (EL3-EL0)
+ * SYS_LED[7:3] - Exception Class (Sync/Async & origin)
+ *
+ */
+#define SYS_LED_SS_SHIFT		0x0
+#define SYS_LED_EL_SHIFT		0x1
+#define SYS_LED_EC_SHIFT		0x3
+
+
 /*******************************************************************************
  * BL2 specific defines.
  ******************************************************************************/