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>