sunxi: OrangePi Zero 2: Enable SPI booting

sunxi: OrangePi Zero 2: Enable SPI booting

The OrangePi Zero 2 board comes with 2MB of SPI flash, from which the
BROM is able to boot from. Please note that the fuse setup requires
PC5 (BOOT_SEL3) to be pulled to GND for that to actually work.

Enable the SPL code responsible for finding and loading U-Boot proper and
friends, so that u-boot-sunxi-with-spl.bin can be written into the flash.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Ivan Shishkin <s45rus@gmail.com>