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/board/sifive/unmatched/MAINTAINERS b/board/sifive/unmatched/MAINTAINERS
new file mode 100644
index 0000000..94c9510
--- /dev/null
+++ b/board/sifive/unmatched/MAINTAINERS
@@ -0,0 +1,9 @@
+SiFive HiFive Unmatched FU740 BOARD
+M:	Paul Walmsley <paul.walmsley@sifive.com>
+M:	Pragnesh Patel <pragnesh.patel@sifive.com>
+M:	Green Wan <green.wan@sifive.com>
+S:	Maintained
+F:	board/sifive/unmatched/
+F:	doc/board/sifive/hifive-unmatched-fu740.rst
+F:	include/configs/sifive-unmatched.h
+F:	configs/sifive_unmatched_defconfig