gpio: dwapb: Read GPIO base from gpio-base property

For backward compatibility with some legacy devices introduce
a new (*) property gpio-base to read GPIO base. This will allow
further cleaning up of the driver.

*) Note, it's not new for the GPIO library since the mockup driver
   is using it already.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>