x86: Change condition for using CAR

At present we assume that CAR (Cache-as-RAM) is used if HOBs (Hand-off
blocks) are not, since HOBs typically indicate that an FSP is in use, and
FSPs handle the CAR init.

However this is a bit indirect, and for FSP2 machines which use their own
CAR implementation (such as apollolake) but use the FSP for other
functions, the logic is wrong.

To fix this, add a dedicated Kconfig option to indicate when CAR is used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: fix a typo in the commit message]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>