Documentation: DT: Add bindings for Spreadtrum SoC Platform

Adds Spreadtrum's prefix "sprd" to vendor-prefixes file.
Adds the devicetree binding documentations for Spreadtrum's sc9836-uart
and SC9836 SoC based on the Sharkl64 Platform which is a 64-bit SoC
Platform of Spreadtrum.

Signed-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>