cbfs: Use void * for the position pointers

It doesn't make sense to use u8 * as the pointer type for accessing the
CBFS since we do not access it as bytes, but via structures. Change it to
void *, which allows us to avoid a cast.

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