rtc: pxa: fix null pointer dereference

With the latest stable kernel versions the rtc on the PXA based
Zaurus does not work, when booting I see the following kernel messages:

pxa-rtc pxa-rtc: failed to find rtc clock source
pxa-rtc pxa-rtc: Unable to init SA1100 RTC sub-device
pxa-rtc: probe of pxa-rtc failed with error -2
hctosys: unable to open rtc device (rtc0)

I think this is because commit f2997775b1 ("rtc: sa1100: fix possible
race condition") moved the allocation of the rtc_device struct out of
sa1100_rtc_init and into sa1100_rtc_probe. This means that pxa_rtc_probe
also needs to do allocation for the rtc_device struct, otherwise
sa1100_rtc_init will try to dereference a null pointer. This patch adds
that allocation by copying how sa1100_rtc_probe in
drivers/rtc/rtc-sa1100.c does it; after the IRQs are set up a managed
rtc_device is allocated.

I've tested this patch with `qemu-system-arm -machine akita` and with a
real Zaurus SL-C1000 applied to 4.19, 5.4, and 5.10.

Signed-off-by: Laurence de Bruxelles <lfdebrux@gmail.com>
Fixes: f2997775b1 ("rtc: sa1100: fix possible race condition")
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220101154149.12026-1-lfdebrux@gmail.com

drivers/rtc/rtc-pxa.c

@@ -330,6 +330,10 @@ static int __init pxa_rtc_probe(struct platform_device *pdev)
 	if (sa1100_rtc->irq_alarm < 0)
 		return -ENXIO;
 
+	sa1100_rtc->rtc = devm_rtc_allocate_device(&pdev->dev);
+	if (IS_ERR(sa1100_rtc->rtc))
+		return PTR_ERR(sa1100_rtc->rtc);
+
 	pxa_rtc->base = devm_ioremap(dev, pxa_rtc->ress->start,
 				resource_size(pxa_rtc->ress));
 	if (!pxa_rtc->base) {