pinctrl: Fix "st,syscfg" definition for STM32 pinctrl

"st,syscfg" entry was bad described.
Signed-off-by: NAlexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

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

@@ -16,7 +16,9 @@ Required properies:
    gpio-bank node (children).
  - interrupt-parent: phandle of the interrupt parent to which the external
    GPIO interrupts are forwarded to.
- - st,syscfg: phandle of the syscfg node used for IRQ mux selection.
+ - 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.