Select CONFIG_SPL_STACK_R for sunxi boards, this gives us much more
room on the stack once we've the DRAM running.

Besides being a good change to have on itself, this also paves the
way for switching to using malloc_simple in the SPL which cuts of
close to 4KiB of the SPL size.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Tom Rini <trini@konsulko.com>