rpi4: Add stdout-path to device tree

Some device tree users like to find a pointer to the standard serial
console in the device tree, in the "stdout-path" property of the /chosen
node.

Add the location of the Mini UART in that property, so that DT users are
happy, for instance Linux' earlycon detection.

Change-Id: I178e55016e5640de5ab0bc6e061944bd3583ea96
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

plat/rpi/rpi4/rpi4_bl31_setup.c

@@ -255,6 +255,9 @@ static void rpi4_prepare_dtb(void)
 	gic_int_prop[2] = cpu_to_fdt32(0x0f04);		// all cores, level high
 	fdt_setprop(dtb, offs, "interrupts", gic_int_prop, 12);
 
+	offs = fdt_path_offset(dtb, "/chosen");
+	fdt_setprop_string(dtb, offs, "stdout-path", "serial0");
+
 	ret = fdt_pack(dtb);
 	if (ret < 0)
 		ERROR("Failed to pack Device Tree at %p: error %d\n", dtb, ret);