soc: esp32xx: refactor clock and RTC subsystems

The RTC subsystem in espressif's SOCs, among other tasks
is responsible for clock selection for CPU and for low
power domain clocks such as RTC_SLOW and RTC_FAST.

This commit allows for proper clock source and rate
selection for CPU, using the espressif,riscv and
espressif,xtensa-lx6/7 bindings.

It also enables clock selection for RTC_FAST and RTC_SLOW,
that impacts some peripherals, such as rtc_timer.

Signed-off-by: Lucas Tamborrino <lucas.tamborrino@espressif.com>