sunxi: Do not define CONFIG_SPL_STACK_ADDR_R

We cannot define a CONFIG value here to ensure that the Kconfig value
isn't set wrong.

Fixes: 2c699fe0 ("configs: sunxi: Add common SUNIV header")
Cc: Icenowy Zheng <icenowy@aosc.io>
Cc: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Tom Rini <trini@konsulko.com>