soc: esp32: set RTC registers to known state

Low-power management is part of the RTC peripherals' domain
on ESP32. This dependency implies the need to bring some RTC
registers to a known state, during system initialization, to
achieve proper low-power handling.

The RTC slow memory region is also delimited and used during
power domain options selection.

Signed-off-by: Glauber Maroto Ferreira <glauber.ferreira@espressif.com>