PM: dpm_watchdog: Add sysctl interface for DPM watchdog timeouts

Introduce sysctl knobs to allow configuring DPM watchdog timeouts at
runtime.

Currently, these timeouts are fixed at compile time via
CONFIG_DPM_WATCHDOG_TIMEOUT and CONFIG_DPM_WATCHDOG_WARNING_TIMEOUT.
This limits flexibility if the timeouts need to be adjusted for
different testing scenarios or hardware behaviors without rebuilding
the kernel.

Add the following sysctl files under /proc/sys/kernel/:
- dpm_watchdog_timeout_secs: The total timeout before panic. The
  maximum value is capped at CONFIG_DPM_WATCHDOG_TIMEOUT to prevent
  unreasonably large timeouts.
- dpm_watchdog_warning_timeout_secs: The warning timeout. The maximum
  value is capped at the current dpm_watchdog_timeout_secs.
Both sysctls have a minimum value of 1.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>