mmc: Add a power-up settling delay in mmc_fdt_set_power()

[manu]

Summary

The SD/SDIO specifications require a settling period after a card is
powered up, before the first command is issued. mmc_fdt_set_power()
enabled the regulators and de-asserted the mmc-pwrseq reset for the
power_up state but returned immediately, so the MMCCAM probe sent
CMD0/CMD5 with no delay. Linux applies an equivalent delay in its MMC
power-up path; MMCCAM had none.

Some devices tolerate this, but others do not respond to the initial
commands if probed too early and then fail to enumerate. On a RockPro64
the Broadcom BCM4359 SDIO Wi-Fi enumerates only intermittently without a
delay; adding one makes it enumerate reliably with no board-specific
post-power-on-delay-ms in the DTS.

Delay for a fixed settling time at the end of the power_up case. This is
a controller-independent fix in the shared FDT MMC helper, so it benefits
all MMCCAM hosts that use mmc_fdt_set_power().

MFC after: 1 week

[mmel]

I don't understand why we need this. We already have a delay (2ms) at the right place, at the end of the mmc_power_up(() function. The MMC/SD/SDIO specification requires 1 ms or 64 clock periods (+10 for stabilisation), thus totalling 0.2 ms, after power line is in range(2.7 - 3.3V) Are you sure that the regulators on the given board have the correct startup ramp constant defined?