sunxi: licheepi_nano: enable SPI flash

sunxi: licheepi_nano: enable SPI flash

Many LicheePi Nano boards come with SPI flash soldered, which already
works for booting the SPL and loading U-Boot proper.
With the updated DTB, we can now also use the SPI flash from U-Boot
proper, so enable the bits in the defconfig, to allow loading binaries
from SPI flash.
There seem to be board revisions with a Winbond SPI chip, but also
others with an XTX chip, so include support for both: the actual chip
used will be autodetected.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>