dts: arm: st: f0: refactor stm32f0 timer drivers

Remove TIM6, 7 & 15 from base dtsi, and add TIM6 & 15 to F030x8, TIM7 for
F030xC, TIM15 for F070, TIM6 & 7 for F070xC, TIM6 & 15 to F051, and TIM7
for F071.
Remove TIM2 from F072 and F091 dtsi since it is already included in F031.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>