MFC r353675 from stable-12 (r353651-r353652 from head)...

r353651:
Relax the sdhci(4) check that filters out the 1.8v voltage option unless
the slot is flagged as 'embedded'.

The features related to embedded and shared slots were added in v3.0 of
the sdhci spec. Hardware prior to v3 sometimes supported 1.8v on non-
removable devices in embedded systems, but had no way to indicate that
via the standard sdhci registers (instead they use out of band metadata
such as FDT data).

This change adds the controller specification version to the check for
whether to filter out the 1.8v selection. On older hardware, the 1.8v
option is allowed to remain. On 3.0 or later it still requires the
embedded-slot flag to remain.

This is part of the fix for PR 241301 (eMMC not detected on Beaglebone).
Changes to the sdhci_ti driver are also needed for a full fix.

PR: 241301

r353652:
Revert r351218 (by manu). While the changes in r351218 appear to be (and
should be) correct, they lead to the eMMC on a Beaglebone failing to work
in some situations.

The TI sdhci hardware is kind of strange. The first device inherently
supports 1.8v and 3.3v and the abililty to switch between them, and the
other two devices must be set to 1.8v in the sdhci power control register to
operate correctly, but doing so actually makes them run at 3.3v (unless an
external level-shifter is present in the signal path). Even the 1.8v on the
first device may actually be 3.3v (or any other value), depending on what
voltage is fed to the VDDS1-VDDS7 power supply pins on the am335x chip.

Another strange quirk is that the convention for am335x sdhci drivers in
linux and uboot and the am335x boot ROM seems to be to set the voltage in
the sdhci capabilities register to 3.0v even though the actual voltage is
3.3v. Why this is done is a complete mystery to me, but it seems to be
required for correct operation.

If we had complete modern support for the am335x chip we could get the
actual voltages from the FDT data and the regulator framework. But our
am335x code currently doesn't have any regulator framework support.
Reverting to the prior code will get the popular Beaglebone boards working
again.

This is part of the fix for PR 241301, but also requires r353651 for a
complete fix.

PR: 241301
Discussed with: manu

Approved by: re(kib)