rtc: zynqmp: Restore alarm functionality after kexec transition

During kexec reboots, RTC alarms that are fired during the kernel
transition experience delayed execution. The new kernel would eventually
honor these alarms, but the interrupt handlers would only execute after
the driver probe is completed rather than at the intended alarm time.

This is because pending alarm interrupt status from the previous kernel
is not properly cleared during driver initialization, causing timing
discrepancies in alarm delivery.

To ensure precise alarm timing across kexec transitions, enhance the
probe function to:
1. Clear any pending alarm interrupt status from previous boot.
2. Detect existing valid alarms and preserve their state.
3. Re-enable alarm interrupts for future alarms.

Signed-off-by: Harini T <harini.t@amd.com>
Link: https://lore.kernel.org/r/20250730142110.2354507-1-harini.t@amd.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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