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/dts/uniphier-ph1-ld4-ref.dts b/arch/arm/dts/uniphier-ph1-ld4-ref.dts
index 469bd05..f62916d 100644
--- a/arch/arm/dts/uniphier-ph1-ld4-ref.dts
+++ b/arch/arm/dts/uniphier-ph1-ld4-ref.dts
@@ -59,7 +59,7 @@
 	status = "okay";
 };
 
-/* for U-boot only */
+/* for U-Boot only */
 / {
 	soc {
 		u-boot,dm-pre-reloc;