x86: spl: Add a function to find the text base

It is useful to know the TEXT_BASE value for the image being loaded in
TPL/SPL. Add a new spl_get_image_text_base() function to handle this.

Make use of this in the x86 SPL handler, instead of having the logic
there.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>