rtc: cmos: Use platform_get_irq_optional() in cmos_platform_probe()

rtc: cmos: Use platform_get_irq_optional() in cmos_platform_probe()

The rtc-cmos driver can live without an IRQ and returning an error
code from platform_get_irq() is not a problem for it in general, so
make it call platform_get_irq_optional() in cmos_platform_probe()
instead of platform_get_irq() to avoid a confusing error message
printed by the latter if an IRQ cannot be found for index 0, which
is possible on x86 platforms.

Additionally, on x86, if the IRQ is not defined and the system has
a legacy PIC, hardcode it to RTC_IRQ, which should be safe then (and
which is what the dropped PNP code did).

Fixes: d15f1c2e413e ("ACPI: PNP: Drop CMOS RTC PNP device support")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/linux-acpi/20260303060752.GA2749263@ax162/
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/12857714.O9o76ZdvQC@rafael.j.wysocki
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>