arm64: dts: allwinner: h6: Pine H64: Add SPI flash node

The Pine H64 board comes with SPI flash soldered on the board, connected
to the SPI0 pins (so it can also boot from there).

Add the required SPI flash DT node to describe this.

Unfortunately the SPI CS0 pin collides with the eMMC CMD pin, so we can't
use both eMMC and SPI flash at the same time (the first to claim the pin
would win, the other's probe routine would then fail).

To avoid losing the more useful eMMC device by chance, mark the SPI
device as "disabled" for now. A user or some U-Boot code could fix this
up if needed, for instance if no eMMC has been detected (it's socketed).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>