pinctrl: st: Staticize local symbols

Symbols used only in this file are made static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>