stm32mp1: add plat_panic_handler function

The STM32MP1 implementation of this function will call
plat_report_exception(). It displays more information about the panic
if DEBUG is enabled.
The LR register is also filled with R6 content, which hold the faulty
address. This allows debugger to reconstruct the backtrace.

Change-Id: I6710e8e2ab6658b05c5bbad2f3c545f07f355afb
Signed-off-by: Yann Gautier <yann.gautier@st.com>

plat/st/stm32mp1/stm32mp1_helper.S

@@ -216,6 +216,23 @@ func plat_crash_console_putc
 	b	console_stm32_core_putc
 endfunc plat_crash_console_putc
 
+	/* ----------------------------------------------------------
+	 * void plat_panic_handler(void) __dead2;
+	 * Report exception + endless loop.
+	 *
+	 * r6 holds the address where the fault occurred.
+	 * Filling lr with this value allows debuggers to reconstruct
+	 * the backtrace.
+	 * ----------------------------------------------------------
+	 */
+func plat_panic_handler
+	mrs	r0, cpsr
+	and	r0, #MODE32_MASK
+	bl	plat_report_exception
+	mov	lr, r6
+	b	.
+endfunc plat_panic_handler
+
 #if DEBUG
 .section .rodata.rev_err_str, "aS"
 abort_str: