reset: stih407: Add reset controllers DT bindings

This patch adds softreset, powerdown and picophy reset controllers
DT bindings for the STiH407 SoC.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>