x86: spl: Add weak arch_cpu_init_dm()

arch_cpu_init_dm() might not be implemented by every platform.
Implement a weak version for SPL.

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

arch/x86/lib/spl.c

@@ -15,6 +15,11 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
+__weak int arch_cpu_init_dm(void)
+{
+	return 0;
+}
+
 static int x86_spl_init(void)
 {
 	/*