Add architecture-specific global data

We plan to move architecture-specific data into a separate structure so
that we can make the rest of it common.

As a first step, create struct arch_global_data to hold these fields.
Initially it is empty.

This patch applies to all archs at once. I can split it if this is really
a pain.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/include/asm/global_data.h

@@ -25,6 +25,11 @@
 
 #ifndef	__ASM_GBL_DATA_H
 #define __ASM_GBL_DATA_H
+
+/* Architecture-specific global data */
+struct arch_global_data {
+};
+
 /*
  * The following data structure is placed in some memory wich is
  * available very early after boot (like DPRAM on MPC8xx/MPC82xx, or
@@ -46,6 +51,7 @@ typedef	struct global_data {
 	const void	*fdt_blob;	/* Our device tree, NULL if none */
 	void		**jt;		/* jump table */
 	char		env_buf[32];	/* buffer for getenv() before reloc. */
+	struct arch_global_data arch;	/* architecture-specific data */
 } gd_t;
 
 #include <asm-generic/global_data_flags.h>