reset: stih407: Add softreset, powerdown and picophy controllers

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

With this patch three new devices are registered: -
1. st,stih407-powerdown
2. st,stih407-softreset
3. st,stih407-picophyreset

All three devices use system configuration registers mapped via regmap to
perform the reset or powerdown. The powerdown controller also has
an acknowledgement.

A separate picophy reset controller manages the different reset channels within
the picophy, which have a different polarity to the other system softresets.
Managing these different picophy softreset channels is necessary to correctly
handle resuming from suspend when USB2 devices are plugged into the USB3 port.

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>