drivers: watchdog: Implement ESP32 watchdog driver

Zephyr's watchdog API is badly designed in the sense that it's a 1:1
abstraction on top of whatever Quark D2000 expects for its watchdog,
instead of expecting a generic timeout value.

This implementation tries as much as possible to calculate the watchdog
timeout in a way that's compatible with a Quark D2000 running at 32MHz;
a comment in adjust_timeout() explains this in more detail.

Jira: ZEP-2296
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>