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>
e0a915a178