i2c: designware: Avoid using static data

Drivers are not allowed to use static data since they may be used in SPL
where BSS is not available.

It is possible that driver model may provide support for numbering devices
in the future. But for now, move this to global_data.

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