arm64: dts: imx8mn-evk: add QSPI flash

There is a 32MiB Micron MT25QU256ABA1 serial NOR flash on the EVK board.
Add a device tree node for it.

Tested on a 8MNANOD3L-EVK.

Signed-off-by: Michael Walle <michael@walle.cc>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>