pinctrl: stm32: move gpio irqs binding to optional

stm32 pinctrl driver could be probed even if no interrupt controller
is defined to manage gpio irqs. Entries related to gpio irq management
are moved to optional.

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/st,stm32-pinctrl.txt

@@ -14,11 +14,6 @@ Required properies:
  - #size-cells	: The value of this property must be 1
  - ranges	: defines mapping between pin controller node (parent) to
    gpio-bank node (children).
- - interrupt-parent: phandle of the interrupt parent to which the external
-   GPIO interrupts are forwarded to.
- - st,syscfg: Should be phandle/offset pair. The phandle to the syscon node
-   which includes IRQ mux selection register, and the offset of the IRQ mux
-   selection register.
  - pins-are-numbered: Specify the subnodes are using numbered pinmux to
    specify pins.
 
@@ -37,6 +32,11 @@ Required properties:
 
 Optional properties:
  - reset:	  : Reference to the reset controller
+ - interrupt-parent: phandle of the interrupt parent to which the external
+   GPIO interrupts are forwarded to.
+ - st,syscfg: Should be phandle/offset pair. The phandle to the syscon node
+   which includes IRQ mux selection register, and the offset of the IRQ mux
+   selection register.
 
 Example:
 #include <dt-bindings/pinctrl/stm32f429-pinfunc.h>