Kconfig: fix FIT offset prompt text

The current prompt text for FIT external offset is identical to
SYS_TEXT_BASE which might confuse the users. Provided more accurate
description for the prompt text.

Signed-off-by: Ibai Erkiaga <ibai.erkiaga-elorza@xilinx.com>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/Kconfig b/Kconfig
index 91c1082..da208d6 100644
--- a/Kconfig
+++ b/Kconfig
@@ -278,7 +278,7 @@
 if FIT
 
 config FIT_EXTERNAL_OFFSET
-	hex "Text Base"
+	hex "FIT external data offset"
 	default 0x0
 	help
 	  This specifies a data offset in fit image.