sunxi: Pine H64: Enable SPI booting in defconfig

The Pine H64 board comes with some onboard SPI flash chip, which is
perfect for loading SPL, ATF and U-Boot proper from there.

Enable the functionality in the defconfig, so that we use sunxi-fel to
transfer a bootable image to the NOR flash.

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