arm64: dts: imx8mn-beacon: Fix SPI CS pinmux

The final production baseboard had a different chip select than
earlier prototype boards.  When the newer board was released,
the SPI stopped working because the wrong pin was used in the device
tree and conflicted with the UART RTS. Fix the pinmux for
production boards.

Fixes: 36ca3c8c

 ("arm64: dts: imx: Add Beacon i.MX8M Nano development kit")
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>