platform: Add T-head C910 initial support

This commit provides basic support for the Thead/C910 platform.

Signed-off-by: Liu Yibin <yibin_liu@c-sky.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

docs/platform/thead-c910.md

@@ -0,0 +1,34 @@
+T-HEAD C910 Processor
+=====================
+C910 is a 12-stage, 3 issues, 8 executions, out-of-order 64-bit RISC-V CPU which
+supports 16 cores, runs with 2.5GHz, and is capable of running Linux.
+
+To build platform specific library and firmwares, provide the *PLATFORM=thead/c910*
+parameter to the top level make command.
+
+Platform Options
+----------------
+
+The *T-HEAD C910* platform does not have any platform-specific options.
+
+Building T-HEAD C910 Platform
+-----------------------------
+
+```
+make PLATFORM=thead/c910
+```
+
+Booting T-HEAD C910 Platform
+-----------------------------
+
+**No Payload**
+
+As there's no payload, you may download vmlinux or u-boot to FW_JUMP_ADDR which
+specified in config.mk or compile commands with GDB. And the execution flow will
+turn to vmlinux or u-boot when opensbi ends.
+
+**Linux Kernel Payload**
+
+You can also choose to use Linux kernel as payload by enabling FW_PAYLOAD=y
+along with specifying FW_PAYLOAD_OFFSET. The kernel image will be embedded in
+the OPENSBI firmware binary, T-head will directly boot into Linux after OpenSBI.