pinctrl: abx500: suppress unused mutex

Suppress a mutex which was initialized on probe
and destroyed on remove but never used.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>