gpio: mcp23s08: handle default gpio base

Create default gpio base if neither device node nor
platform data is defined.

Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Antonio Fiol <antonio@fiol.es>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>