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/drivers/mtd/nand/mxc_nand_spl.c b/drivers/mtd/nand/mxc_nand_spl.c
index 69b736a..6ac2c96 100644
--- a/drivers/mtd/nand/mxc_nand_spl.c
+++ b/drivers/mtd/nand/mxc_nand_spl.c
@@ -337,7 +337,7 @@
 	if (!nand_spl_load_image(CONFIG_SYS_NAND_U_BOOT_OFFS,
 			CONFIG_SYS_NAND_U_BOOT_SIZE,
 			(uchar *)CONFIG_SYS_NAND_U_BOOT_DST)) {
-		/* Copy from NAND successful, start U-boot */
+		/* Copy from NAND successful, start U-Boot */
 		uboot = (void *)CONFIG_SYS_NAND_U_BOOT_START;
 		uboot();
 	} else {