sunxi: A64: move SPL stack to end of SRAM A2

The SPL stack is usually located at the end of SRAM A1, where it grows
towards the end of the SPL.
For the really big AArch64 binaries the stack overwrites code pretty
soon, so move the SPL stack to the end of SRAM A2, which is unused at this
time.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Jagan Teki <jagan@openedev.com>