vexpress64: Enable OF_CONTROL and OF_BOARD for VExpress64

Capture x0 in lowlevel_init.S as potential fdt address. Modify
board_fdt_blob_setup to use fdt address from either vexpress_aemv8.h
or lowlevel_init.S.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/board/armltd/vexpress64/lowlevel_init.S b/board/armltd/vexpress64/lowlevel_init.S
new file mode 100644
index 0000000..3dcfb85
--- /dev/null
+++ b/board/armltd/vexpress64/lowlevel_init.S
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * (C) Copyright 2021 Arm Limited
+ */
+
+.global save_boot_params
+save_boot_params:
+
+	adr	x8, prior_stage_fdt_address
+	str	x0, [x8]
+
+	b	save_boot_params_ret