drivers: wdt: espressif: Add 32K Xtal Watchdog

This WDT is responsible for monitoring the external
32.728 Hz crystal connected to pins XTAL_32K_P and
XTAL_32K_N. If an oscillation failure is detected
the hardware automatically switch to RTC_RC_SLOW
clock source and triggers an interrupt.

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