spl: introduce SPL_XIP to config

U-Boot and SPL don't necessary share the same location, so we might end
with U-Boot SPL in read-only memory (XIP) and U-Boot in read-write memory.

In case of non XIP boot mode, we rely on such variables as "hart_lottery"
and "available_harts_lock" which we use as atomics.

The problem is that CONFIG_XIP also propagate to main U-Boot, not only SPL,
so we need CONFIG_SPL_XIP to distinguish SPL XIP from other XIP modes.

This adds an option special for SPL to behave it in XIP manner and we don't
use hart_lottery and available_harts_lock, during start proccess.

Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Rick Chen <rick@andestech.com>