gpio: mcp23s08: make driver depend on OF_GPIO

The MCP23S08 driver certainly accesses fields inside the
struct gpio_chip that are only available under CONFIG_OF_GPIO
not just CONFIG_OF, so update the Kconfig and driver to reflect
this.

Cc: Alexander Stein <alexander.stein@systec-electronic.com>
Cc: Phil Reid <preid@electromag.com.au>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>