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>