i2c: designware: Move reg-space remapping into a dedicated function

This is a preparation patch before adding a quirk with custom registers
map creation required for the Baikal-T1 System I2C support.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>