Use correct spelling of "U-Boot"

Correct spelling of "U-Boot" shall be used in all written text
(documentation, comments in source files etc.).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Minkyu Kang <mk7.kang@samsung.com>
diff --git a/arch/blackfin/include/asm/io.h b/arch/blackfin/include/asm/io.h
index aadb0d2..d3337e4 100644
--- a/arch/blackfin/include/asm/io.h
+++ b/arch/blackfin/include/asm/io.h
@@ -1,5 +1,5 @@
 /*
- * U-boot - io.h IO routines
+ * U-Boot - io.h IO routines
  *
  * Copyright 2004-2009 Analog Devices Inc.
  *