pinctrl: stm32: add syscfg mask parameter

This patch adds mask parameter to define IRQ mux field.
This field could vary depend of IRQ mux selection register.
To avoid backward compatibility, the drivers set
the legacy value by default.

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>