pinctrl: st: Annotate st_pinconf_dbg_show() as holding mutex

pinctrl: st: Annotate st_pinconf_dbg_show() as holding mutex

st_pinconf_dbg_show() unlocks and locks the pinctrl_dev mutex, so it
must be called by the pinctrl core with the mutex hold.  Annotate the
function with sparse __must_hold, so any changes in pinctrl locking will
be statically detected.

Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>