drivers: reset: stih415: add softreset controller

This patch adds softreset controller for STiH415 SOC, soft reset
controller is based on system configuration registers which are mapped
via regmap. This reset controller does not have any feedback or
acknowledgement. With this patch a new device "st,stih415-softreset" is
registered with system configuration registers based reset controller
that controls the softreset state of the hardware such as Ethernet, IRB.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>