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>
diff --git a/arch/riscv/cpu/cpu.c b/arch/riscv/cpu/cpu.c
index 3ffcbbd..0f323b2 100644
--- a/arch/riscv/cpu/cpu.c
+++ b/arch/riscv/cpu/cpu.c
@@ -19,7 +19,7 @@
  * The variables here must be stored in the data section since they are used
  * before the bss section is available.
  */
-#ifndef CONFIG_XIP
+#if !CONFIG_IS_ENABLED(XIP)
 u32 hart_lottery __section(".data") = 0;
 
 /*