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/board/sifive/unleashed/MAINTAINERS b/board/sifive/unleashed/MAINTAINERS
new file mode 100644
index 0000000..2ea0074
--- /dev/null
+++ b/board/sifive/unleashed/MAINTAINERS
@@ -0,0 +1,10 @@
+SiFive HiFive Unleashed BOARD
+M:	Paul Walmsley <paul.walmsley@sifive.com>
+M:	Palmer Dabbelt <palmer@dabbelt.com>
+M:	Anup Patel <anup.patel@wdc.com>
+M:	Atish Patra <atish.patra@wdc.com>
+S:	Maintained
+F:	board/sifive/unleashed/
+F:	doc/board/sifive/unleashed.rst
+F:	include/configs/sifive-unleashed.h
+F:	configs/sifive_unleashed_defconfig