sunxi: OrangePi Zero: defconfig: enable SPI flash

Newer OrangePi Zero boards all come with 16 Mib SPI flash soldered, from
which the board can actually boot from.
Enable the SPL support for the SPI controller and SPI flash to allow
putting the SPL, the DT and U-Boot proper into there. This will let
a board boot without an SD card inserted.
The flash chip can be written with a version of the sunxi-fel tool.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Priit Laes <plaes@plaes.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>