This will avoid programming the retime registers when not implemented

- PIO5  : no retime registers assigned to pins 6 and 7
- PIO35 : pin 7 is reserved so no retime register assigned to it
Signed-off-by: Karim BEN BELGACEM <karim.ben-belgacem@st.com>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>