Fix the "pending" state of RTCs, on Pi 5 and beyond

[pelwell]

Investigating a report of the Pi 5 RTC not correctly advertising the alarm "pending" state - an indication that the alarm has fired and woken the host - appears to show that there are bugs in the Pi 5 RTC implementation, but also that it is broken for all RTCs.

These two patches fix the rtc-pi5 bug (a simple failure to populate the pending field of struct rtc_wkalrm), but also modify the RTC framework such that the value read from the RTCs is made available to users of rtc_read_alaram, including the code that provides the output from /proc/driver/rtc.

See: raspberrypi/firmware#2011

[pelwell]

N.B. Fixing the Pi 5 RTC case also requires an updated EEPROM, a trial build of which can be downloaded from here: https://drive.google.com/file/d/1Wsd0pl9WRiN6BRyxsKEn1shA2GZoFQSq/view?usp=sharing

Once downloaded as pieeprom_26015.bin, install it like this:

$ sudo rpi-eeprom-config -o eeprom-config.txt
$ sudo rpi-eeprom-config -c eeprom-config.txt --out pieeprom.bin pieeprom_260116.bin
$ sudo rpi-eeprom-update -f pieeprom.bin

The first two commands extract your current EEPROM config settings and apply them to new EEPROM image, and the third writes it to the EEPROM.

Installing the trial kernel build is easier - just sudo rpi-update pulls/7198, but be sure to read the caveats first: https://github.com/raspberrypi/rpi-update?tab=readme-ov-file#caveats