The Rockchip pinctrl driver needs to handle information from Devicetree
so only makes sense getting compiled on systems with CONFIG_OF enabled.

This also fixes a problem found by the "kernel-test-robot" when compiling
the driver on test-builds that do not have CONFIG_OF enabled:

  drivers/pinctrl/pinctrl-rockchip.c: In function 'rockchip_pinctrl_parse_groups':
>> drivers/pinctrl/pinctrl-rockchip.c:2881:9: error: implicit declaration of function 'pinconf_generic_parse_dt_config'; did you mean 'pinconf_generic_dump_config'? [-Werror=implicit-function-declaration]
    2881 |   ret = pinconf_generic_parse_dt_config(np_config, NULL,
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         |         pinconf_generic_dump_config
   drivers/pinctrl/pinctrl-rockchip.c: In function 'rockchip_gpiolib_register':
>> drivers/pinctrl/pinctrl-rockchip.c:3473:5: error: 'struct gpio_chip' has no member named 'of_node'
    3473 |   gc->of_node = bank->of_node;
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20200905214955.907950-1-heiko@sntech.deSigned-off-by: Linus Walleij <linus.walleij@linaro.org>