DTS: Add imx6q-display5-u-boot.dtsi file with u-boot specific properties

This file setups UART5 based serial to be used as pre-relocation
console in the U-Boot proper.

On purpose pinux configuration is omitted here as it has been already
done in SPL. For early pre-relocation code we only need the serial
device from DTS.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>