arm64: dts: imx8mm-beacon: Fix ecspi2 pinmux

Early hardware did not support hardware handshaking on the UART, but
final production hardware did.  When the hardware was updated the chip
select was changed to facilitate hardware handshaking on UART3.  Fix the
ecspi2 pin mux to eliminate a pin conflict with UART3 and allow the
EEPROM to operate again.

Fixes: 4ce01ce3 ("arm64: dts: imx8mm-beacon: Enable RTS-CTS on UART3")
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>