gpio: dwapb: Use "stride" rather than "size" for register distance

This terminology is more precise. Also cut the stride calculation
in the preprocessor, it confuses more than it helps when reading
the driver.

Acked-by: Hoan Tran <hotran@apm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>