arm64: dts: rockchip: enable onboard spi flash for rock-3a

There is a mx25u12835f spi flash on this board, enable it.

[    2.525805] spi-nor spi4.0: mx25u12835f (16384 Kbytes)

Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
Link: https://lore.kernel.org/r/20240409120003.309358-2-amadeus@jmu.edu.cn


Signed-off-by: Heiko Stuebner <heiko@sntech.de>