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>

Documentation/devicetree/bindings/arm/sprd.txt

+Spreadtrum SoC Platforms Device Tree Bindings
+----------------------------------------------------
+
+Sharkl64 is a Spreadtrum's SoC Platform which is based
+on ARM 64-bit processor.
+
+SC9836 openphone board with SC9836 SoC based on the
+Sharkl64 Platform shall have the following properties.
+
+Required root node properties:
+        - compatible = "sprd,sc9836-openphone", "sprd,sc9836";

Documentation/devicetree/bindings/serial/sprd-uart.txt

+* Spreadtrum serial UART
+
+Required properties:
+- compatible: must be "sprd,sc9836-uart"
+- reg: offset and length of the register set for the device
+- interrupts: exactly one interrupt specifier
+- clocks: phandles to input clocks.

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -154,6 +154,7 @@ snps	Synopsys, Inc.
 solidrun	SolidRun
 sony	Sony Corporation
 spansion	Spansion Inc.
+sprd	Spreadtrum Communications Inc.
 st	STMicroelectronics
 ste	ST-Ericsson
 stericsson	ST-Ericsson