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/mips/cpu/start.S b/arch/mips/cpu/start.S
index d2c31ae..1b56ca3 100644
--- a/arch/mips/cpu/start.S
+++ b/arch/mips/cpu/start.S
@@ -57,7 +57,7 @@
 	.set noreorder
 
 ENTRY(_start)
-	/* U-boot entry point */
+	/* U-Boot entry point */
 	b	reset
 	 nop