arm64: dts: Add dts files for LG Electronics's lg1312 SoC

Add initial dtsi file to support lg1312 SoC which based on
Cortex-A53. Also add dts file to support lg1312 reference board
which based on lg1312 SoC.

Signed-off-by: Chanho Min <chanho.min@lge.com>
Signed-off-by: Olof Johansson <olof@lixom.net>