gpio: vf610: allow disabling the vf610 driver

The vf610 gpio driver is enabled by default for all i.MX machines,
without any option to disable it in a board-specific config file.

Most i.MX chipsets have no hardware for this driver. Change the default
to enable GPIO_VF610 for SOC_VF610 and disable it otherwise.

Add a text description after the bool type, this makes the driver
selectable by make config etc.

Fixes: 30a35c07 ("gpio: vf610: drop the SOC_VF610 dependency for GPIO_VF610")
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>