x86: Add some missing global_data declarations in files that use gd

Some files use global_data but don't declare it. Fix this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/x86/cpu/mp_init.c b/arch/x86/cpu/mp_init.c
index e686b28..3294a50 100644
--- a/arch/x86/cpu/mp_init.c
+++ b/arch/x86/cpu/mp_init.c
@@ -24,6 +24,8 @@
 #include <dm/uclass-internal.h>
 #include <linux/linkage.h>
 
+DECLARE_GLOBAL_DATA_PTR;
+
 /* Total CPUs include BSP */
 static int num_cpus;