board: sifive: add HiFive Unmatched board support

Add defconfig and board support for HiFive Unmatched.

Signed-off-by: Green Wan <green.wan@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/doc/board/sifive/index.rst b/doc/board/sifive/index.rst
index ed7eacf..a43937a 100644
--- a/doc/board/sifive/index.rst
+++ b/doc/board/sifive/index.rst
@@ -7,3 +7,4 @@
    :maxdepth: 2
 
    unleashed
+   unmatched