nRF52: watchdog timeout is not accurate

[XuGuohui]

Problem

As the title describes. The method of calculating the watchdog reload value may cause the intermediate value overflow, which results in an incorrect reload value being loaded to register.

Solution

Recent nRF5 SDK has fixed this. The PR simply extracts the watchdog related files from the recent SDK and replace the existing files.

Example App

#include "application.h"

SYSTEM_MODE(MANUAL);
SerialLogHandler l(LOG_LEVEL_ALL);

void setup() {
    while (!Serial.isConnected());
    Watchdog.init(WatchdogConfiguration().timeout(240s));
    Watchdog.start();
}

void loop() {
    delay(1s);
    Log.info("Running...");
}

References

https://community.particle.io/t/hardware-watchdog-in-device-os-5-3-0/64031/16

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[avtolstoy]

Nit: I'd generalize this test probably and just move the platform-dependent functionality to get current reload value etc into a separate function.