armv8: spl: Call spl_relocate_stack_gd for ARMv8

As part of the startup process for boards using the SPL, we need to
call spl_relocate_stack_gd. This is needed to set up malloc with its
DRAM buffer.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/lib/crt0_64.S

@@ -109,8 +109,18 @@ relocation_return:
  */
 	bl	c_runtime_cpu_setup		/* still call old routine */
 #endif /* !CONFIG_SPL_BUILD */
-
-/* TODO: For SPL, call spl_relocate_stack_gd() to alloc stack relocation */
+#if defined(CONFIG_SPL_BUILD)
+	bl	spl_relocate_stack_gd           /* may return NULL */
+	/*
+	 * Perform 'sp = (x0 != NULL) ? x0 : sp' while working
+	 * around the constraint that conditional moves can not
+	 * have 'sp' as an operand
+	 */
+	mov	x1, sp
+	cmp	x0, #0
+	csel	x0, x0, x1, ne
+	mov	sp, x0
+#endif
 
 /*
  * Clear BSS section