Conscript TIM5 for early boot

[jamesmunns]

This change implements a very simple hardware timer using the 32-bit TIM5.

I'm open to better ideas of how to test this, but using the HUBRIS_MEASUREMENT_RESULT as a signal:

# Current `master`, measured on a grapefruit
$ humility -d ./hubris.core.0 readvar HUBRIS_MEASUREMENT_RESULT
humility: attached to dump
HUBRIS_MEASUREMENT_RESULT (0x24000404) = Some(Valid {
        count: Some(0x7)
    })

# This change, as-is (20x200ms delays)
$ humility -d ./hubris.core.1 readvar HUBRIS_MEASUREMENT_RESULT
humility: attached to dump
HUBRIS_MEASUREMENT_RESULT (0x24000404) = Some(Valid {
        count: Some(0xd)
    })

# This change, but with `DELAY_MICROS` set to `1_000 * 1_000`, or 20x1s delays
$ humility -d ./hubris.core.2 readvar HUBRIS_MEASUREMENT_RESULT
humility: attached to dump
HUBRIS_MEASUREMENT_RESULT (0x24000404) = Some(Valid {
        count: Some(0x3)
    })

This PR is intended as a lay-up to #2506, though for that future use we would need to:

1. Re-configure and re-enable the timer for the increased APB1L frequency (100MHz, instead of 64MHz)
2. Implement a "get current rolling time" free function, provide that to the kernel
3. Find a way to tell the kernel the range and frequency of the rolling timer (e.g. 32-bit, 1MHz)
4. Implement the embassy-time "rolling timer extension" algorithm described in Consider requiring a higher resolution platform-specific timer #2506
5. (optional) add a "get micros since boot -> u64" syscall for use in userspace

[labbott]

I'll confess to initially being skeptical of using timers but this looks really nice. I'll take a closer look later this week.

[labbott]

It would be good to do at least one boot/check on a gimlet for the revision strap pins since debugging that was awful.

[jamesmunns]

It would be good to do at least one boot/check on a gimlet for the revision strap pins since debugging that was awful.

I've now tested this on:

• gimlet-d
• sidecar-b
• psc-c

Using both power-off resets (using niles' power control), and with soft resets (humility reset). I wasn't able to test a minibar, but it seems to use the same delay timing as sidecar.

Let me know if you'd like me to chase down a minibar as well, or if this is close enough for comfort. Either way, I might hack together a test on Monday to toggle a pin using the timer and watch it with a saleae, just to make sure that it really is ticking at 64MHz as I expect.