ARM: dts: sunxi: Fix the SPI NOR node names

According to the SPI NOR bindings, the flash node names are supposed to
be flash@<address>. Let's fix our users to use that new scheme.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20210901091852.479202-44-maxime@cerno.tech