i2c: designware_i2c: Move dw_i2c_speed_config to header

This is used to store the speed information for a bus. We want to provide
this to ACPI so that it can tell the kernel. Move this struct to the
header file so it can be accessed by the ACPI i2c implementation being
added later.

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