rtc: interface: Add rtc_read_next_alarm() to read next expiring timer

Add a new function rtc_read_next_alarm() that reads the next expiring
alarm from the RTC timerqueue. This is different from rtc_read_alarm(),
which only reads the aie_timer.

The wakealarm sysfs file programs the rtc->aie_timer, whereas the
alarmtimer suspend routine programs its own timer into the RTC timerqueue.
Both timers end up in the RTC's timerqueue, and the first expiring timer
is what gets armed in the hardware.

This new function allows code to query which alarm will actually fire
next, regardless of which subsystem programmed it. This is needed by
platform code that needs to program secondary timers based on the
actual next wakeup time.

Link: https://lore.kernel.org/all/87ed50z0le.ffs@tglx
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://patch.msgid.link/20260521043714.1022930-2-mario.limonciello@amd.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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