x86: Add an option to control the position of SPL

x86: Add an option to control the position of SPL

For Apollo Lake SPL is run from CAR (cache-as-RAM) which is in a different
location from where SPL must be placed in ROM. In other words, although
SPL runs before SDRAM is set up, it is not execute-in-place (XIP).

Add a Kconfig option for the ROM position.

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