[hal] sleep: allow to enter hibernate mode without specifying any wak…

[XuGuohui]

Problem

Gen3 device cannot enter hibernate mode if not specifying any wakeup source.

Solution

Don't check wakeup source availability in sleep HAL if hibernate mode is specified.

Example App

void setup() {
    System.sleep(SLEEP_MODE_DEEP, SLEEP_DISABLE_WKP_PIN);
    // or
    // System.sleep(SystemSleepConfiguration().mode(SystemSleepMode::HIBERNATE));
    // Device should enter hibernate mode here. And it can only be woken up by a reset or power-cycle.
}

void loop() {
}

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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)