sunxi: Select SPL_SEPARATE_BSS

sunxi-common.h defines CONFIG_SPL_BSS_START_ADDR to put SPL's BSS in
DRAM. Due to this, we must select SPL_SEPARATE_BSS, or else SPL will
attempt to load its DTB from the wrong address (after BSS in DRAM).

This change fixes booting with SPL_OF_CONTROL=y.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>