soc: espressif: convert rtc peripheral to clock subsystem

Current ESP32 clock system is mixed with RTC labeling/registers,
but it doesn't implement a real-time clock (RTC) driver.

To avoid confusion and allow adding a proper RTC driver later,
this commit renames the existing RTC interface to CLOCK and make
it as a subsystem without any peripheral attached to it.

This better reflects its actual purpose as a general clock controller.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>