arm64: dts: Add basic DT to support Spreadtrum's SP9860G

SC9860G is a 8 cores of A53 SoC with 4G LTE support SoC from Spreadtrum.

According to regular hierarchy of sprd dts, whale2.dtsi contains SoC
peripherals IP nodes, sc9860.dtsi contains stuff related to ARM core stuff
and sp9860g dts is for the board level.

Signed-off-by: Orson Zhai <orson.zhai@spreadtrum.com>
Signed-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>