ARM: provide a valid exception stack address for startup code

Create exception stack in IRAM if available to facilitate debugging of
pre-relocation code by catching exceptions rather than stopping dead.
Signed-off-by: NLothar Waßmann <LW@KARO-electronics.de>

arch/arm/lib/vectors.S

@@ -117,7 +117,6 @@ data_abort:
 not_used:
 irq:
 fiq:
-
 1:
 	bl	1b			/* hang and never return */
 
@@ -126,7 +125,11 @@ fiq:
 /* IRQ stack memory (calculated at run-time) + 8 bytes */
 .globl IRQ_STACK_START_IN
 IRQ_STACK_START_IN:
+#ifdef IRAM_BASE_ADDR
+	.word   IRAM_BASE_ADDR + 0x20
+#else
 	.word	0x0badc0de
+#endif
 
 @
 @ IRQ stack frame.