x86: Set up a shared syscon numbering schema

x86: Set up a shared syscon numbering schema

Each system controller can have a number to identify it. It can then be
accessed using syscon_get_by_driver_data(). Put this in a shared header
file and update the only current user.

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