sdhci_xenon: improve the VCCQ voltage switch sequence

[mw]

Summary

Improve the VCCQ voltage switch, so that to properly
handle the SDHCI_HOST_CONTROL2 register signaling
flags, along with manipulating the regulator.

[manu]

sys/dev/sdhci/sdhci_xenon.c

425

Is this delay needed ? The regulator set_voltage function should write to the delay variable and the regulator framework will sleep.

[mw]

sys/dev/sdhci/sdhci_xenon.c

425

According to SD Host Controller Simplified Specification 4.20: 'Host driver should take more than 5ms for stable time of host voltage regulator from changing 1.8V Signaling Enable. Specific host driver may use a specific time, which is provided by host system, instead of using 5ms.'

I prefer to keep it explicitly here, even if regulator_set_voltage adds its own. Hint - Linux and EDK2 do it in generic code, it works and I'd rather not deviate :)

[manu]

sys/dev/sdhci/sdhci_xenon.c

425

Could you add a comment quoting the spec then please ?
(Same for below)