mmccam: Wait for the power to settle when probing a card

[manu]

Summary

The mmc(4) stack waits after powering up a card: mmc_power_up() waits 1 ms
after the supplies are on and 2 ms after the clock is running, before the
first command is sent. The MMCCAM probe had no such wait: it went from the
power_up request to the power_on request to CMD0 without any delay.

Some devices do not answer the first commands if they are sent too early and
then never enumerate, with no error. On a RockPro64 the Broadcom SDIO Wi-Fi
(BCM4359) was not detected on any of 8 boots without a delay. It was
detected on 8 out of 8 with the same waits as mmc_power_up() added to the
probe.

Add the same two waits, once the power_up request and once the power_on
request are done. This replaces the delay that was in mmc_fdt_set_power(),
which was a fixed 10 ms for FDT hosts only; the waits of the legacy stack are
enough and apply to every MMCCAM host.

MFC after: 1 week

[imp]

I'm going to click OK here... but this can lead to 1ms or 2ms delays in processing other CAM devices since it's a busy wait. For MMC, that's likely OK. They don't usually have other devices on the system that are so performance critical that a stall like this would matter... However, as usage shifts, we may need to revisit one day. For now I think this is OK, with my mentioning the caveat in case people trace some future performance problem back to this commit and then to this review.

To be clear, I'm not sure doing two extra states is worth it today, and this change should go in.