sandbox: Allow use of real I/O with readl(), etc.

At present these functions are stubbed out. For more comprehensive testing
with PCI devices it is useful to be able to fully emulate I/O access. Add
simple implementations for these.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: change to use 'const void *' in sandbox_write();
        cast 'addr' in read/write macros in arch/sandbox/include/asm/io.h;
        remove the unnecessary cast in readq/writeq in nvme.h]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/sandbox/include/asm/state.h b/arch/sandbox/include/asm/state.h
index 2d773d3..ad3e94b 100644
--- a/arch/sandbox/include/asm/state.h
+++ b/arch/sandbox/include/asm/state.h
@@ -102,6 +102,7 @@
 	ulong next_tag;			/* Next address tag to allocate */
 	struct list_head mapmem_head;	/* struct sandbox_mapmem_entry */
 	bool hwspinlock;		/* Hardware Spinlock status */
+	bool allow_memio;		/* Allow readl() etc. to work */
 
 	/*
 	 * This struct is getting large.