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-exynos/sec_boot.S b/arch/arm/mach-exynos/sec_boot.S
index dfc3455..5dc216d 100644
--- a/arch/arm/mach-exynos/sec_boot.S
+++ b/arch/arm/mach-exynos/sec_boot.S
@@ -30,10 +30,10 @@
  * because that comes out to be the last 4KB of the iRAM
  * (Base Address - 0x02020000, Limit Address - 0x020740000).
  *
- * U-boot and kernel are aware of this code and flags by the simple
+ * U-Boot and kernel are aware of this code and flags by the simple
  * fact that we are implementing a workaround in the last 4KB
  * of the iRAM and we have already defined these flag and address
- * values in both kernel and U-boot for our use.
+ * values in both kernel and U-Boot for our use.
  */
 code_base:
 	b	 1f