arm64: dts: allwinner: sun50i: h5: Add spi flash node for OrangePi PC2

The OrangePi PC2 have an mx25l1606e spi flash.
Add a node for it.

Signed-off-by: Emmanuel Vadot <manu@freebsd.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>