STM32U5 low-power: time driver with tick-hz-1_000_000 wrong; with 32_678 correct

[chrenderle]

During testing for #3499 I discovered that when the time driver tick frequency is set to 1_000_000 the time is wrong when using the low-power feature.

I have a timeout task set to busy-loop after 60s. When the tick frequency is set to 32_678 it works as expected. The mcu wakes up after 32s because of the current rtc limitations. It then goes to sleep for another 28s and then the timeout task blocks.

With the tick frequency set to 1_000_000 the mcu runs for ~2m21s. The first two times it wakes up after 32s but thinks it only slept 16s. After that it sleeps for 27s and 3x16s. After that the timeout activates.

[chrenderle]

My initial testing was done on a Nucleo-U575ZI-Q. I could confirm the problem on a Nucleo-L552ZE-Q too, although it only runs for 1m48s there, not 2m21s.