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/arm/mach-orion5x/lowlevel_init.S b/arch/arm/mach-orion5x/lowlevel_init.S
index 51a8b3c..3f38f36 100644
--- a/arch/arm/mach-orion5x/lowlevel_init.S
+++ b/arch/arm/mach-orion5x/lowlevel_init.S
@@ -283,5 +283,5 @@
 
 #endif /* CONFIG_SPL_BUILD */
 
-	/* Return to U-boot via saved link register */
+	/* Return to U-Boot via saved link register */
 	mov	pc, lr