Documentation: gpio: driver: Format code blocks properly

This fixes a lot of Sphinx warnings, and makes the code blocks look nice
in HTML.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

Documentation/driver-api/gpio/driver.rst

@@ -415,6 +415,8 @@ If you do this, the additional irq_chip will be set up by gpiolib at the
 same time as setting up the rest of the GPIO functionality. The following
 is a typical example of a cascaded interrupt handler using gpio_irq_chip:
 
+.. code-block:: c
+
   /* Typical state container with dynamic irqchip */
   struct my_gpio {
       struct gpio_chip gc;
@@ -450,6 +452,8 @@ is a typical example of a cascaded interrupt handler using gpio_irq_chip:
 The helper support using hierarchical interrupt controllers as well.
 In this case the typical set-up will look like this:
 
+.. code-block:: c
+
   /* Typical state container with dynamic irqchip */
   struct my_gpio {
       struct gpio_chip gc;