riscv: sifive: Rename fu540 board to unleashed

In preparation to add SiFive Unmatched board support, let's rename
the existing fu540 board to unleashed.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/doc/board/sifive/index.rst b/doc/board/sifive/index.rst
index ad614c9..ed7eacf 100644
--- a/doc/board/sifive/index.rst
+++ b/doc/board/sifive/index.rst
@@ -6,4 +6,4 @@
 .. toctree::
    :maxdepth: 2
 
-   fu540
+   unleashed
diff --git a/doc/board/sifive/fu540.rst b/doc/board/sifive/unleashed.rst
similarity index 100%
rename from doc/board/sifive/fu540.rst
rename to doc/board/sifive/unleashed.rst