reset: stm32: migrate restart reason and handler from stm32_iwdg ...

The watchdog driver will remain unprobed if the driver or the OF node
is disabled, but the reset reason is useful even then.

System reset and reset source determination is achieved with the RCC
peripheral for which we have a reset controller driver.
Move the code over there, so reset reason and reset are available
always.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

reset: add reset controller driver for STM32 RCC ...

On the STM32MP, reset of the I2C, SPI and USB IPs occurs over the RCC.
This driver adds support for the controller, so it may be reused by
other drivers.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>