imx6sx-udoo-neo-basic: Introduce the u-boot.dtsi

imx6sx-udoo-neo-basic: Introduce the u-boot.dtsi

After the conversion to DM_SERIAL in commit 4de0f15ee44d ("udoo_neo:
Select DM_SERIAL and drop iomux board level init") the SPL log is gone
and the U-Boot proper log becomes incomplete:

Core:  80 devices, 18 uclasses, devicetree: separate
MMC:   FSL_SDHC: 1, FSL_SDHC: 2
Loading Environment from MMC... OK
In:    serial@2020000
Out:   serial@2020000
Err:   serial@2020000
Net:   eth0: ethernet@2188000
Hit any key to stop autoboot:  0

Introduce the u-boot.dtsi file that passes the u-boot,dm-pre-reloc
properties to the relevant nodes so that UART can be used early in SPL.

With this change, the complete SPL and U-Boot messages are seen again.

Signed-off-by: Fabio Estevam <festevam@denx.de>