gpio: mcp23s08: handle default gpio base

commit b184c388f773f30b6c707d3d4599b2db80f4390c upstream.

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

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>