ARM: dts: imx6ull-colibri: pre-reloc for uart pinmux modes

Add u-boot,dm-pre-reloc properties for uart pinmux configuration
nodes, which enables UART as early as possible (before relocation).

Without this we miss almost the half of output (U-boot version,
CPU defails, Reset cause, DRAM details etc.).

Fixes: cd69e8ef ("colibri-imx6ull: migrate pinctrl and regulators to dtb/dm")
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>