dt-bindings: pinctrl: add syscfg mask parameter

This patch adds mask parameter to define IRQ mux field.
This field could vary depend of IRQ mux selection register.
This parameter is needed if the mask is different of 0xf.

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