arm: dts: imx8mn-beacon-kit-u-boot: Fix broken booting

When the imx8mm.dtsi file was pulled in from Linux, the UARTs
were moved into an spba sub-node which wasn't being included
in the SPL device tree.  This meant the references to the UART
weren't being handled properly and when booting the system would
constantly reboot.  Fix this by adding the spba node to the spl
device tree to restore normal booting.

Fixes: 4e5114da ("imx8mn: synchronise device tree with linux")
Signed-off-by: Adam Ford <aford173@gmail.com>