RTEMS | score: Fix TOD_LATEST_YEAR and watchdog overflow beyond 2400 (!1484)

[Sebastian Huber (@sebhub)]

Sebastian Huber commented on a discussion: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1484#note_159809


The `uint64_t Watchdog_Control::expire` member represents a time point for all three clocks: ticks, CLOCK_REALTIME, CLOCK_MONOTONIC. The POSIX clocks use 34 bits for the seconds part and 30 bits for the nanoseconds:
```c
/**
 * @brief The maximum watchdog ticks value for the far future.
 */
#define WATCHDOG_MAXIMUM_TICKS UINT64_MAX

#define WATCHDOG_NANOSECONDS_PER_SECOND 1000000000

/**
 * @brief The bits necessary to store 1000000000
 * (= WATCHDOG_NANOSECONDS_PER_SECOND) nanoseconds.
 *
 * The expiration time is an unsigned 64-bit integer.  To store nanoseconds
 * timeouts we use 30 bits (2**30 == 1073741824) for the nanoseconds and 34
 * bits for the seconds since UNIX Epoch.  This leads to a year 2514 problem.
 */
#define WATCHDOG_BITS_FOR_1E9_NANOSECONDS 30

/**
 * @brief The maximum number of seconds representable in the nanoseconds
 * watchdog format.
 *
 * We have 2**34 bits for the seconds part.
 */
#define WATCHDOG_MAX_SECONDS 0x3ffffffff
```

-- 
View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1484#note_159809
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