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>