hwspinlock: stm32: implement the relax() ops

Implement this optional ops, called by hwspinlock core while spinning on
a lock, between two successive invocations of trylock().

Reviewed-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>