MCI bus-width host caps via device tree don't work properly for dw and imx

[Trent Piepho]

I found my 4-bit connected eMMC chip wasn't working because barebox was
trying to run it in 8-bit mode.  I had the proper bus-width property in
the device tree, but this doesn't work.

The problem is that mci_of_parse() uses bus-width to _add_ 8-bit and/or
4-bit caps to the host.  It doesn't remove any host_caps the driver
already has set.  So if the driver supports 8-bit but the DT bus-width
is 4, you still get a driver that thinks 8-bit will work.

You can see this in dw_mmc.c, dw_mmc_probe() sets:
host->mci.host_caps = MMC_CAP_4_BIT_DATA | MMC_CAP_8_BIT_DATA
then later
mci_of_parse(&host->mci);
return mci_register(&host->mci);

The mci_of_parse call will not remove the 4 and 8 bits caps.  There is
the same problem in imx-esdhc.c.  The other two users of mci_of_parse,
mxs.c and tegra-sdmmc.c, don't have this problem, since they don't set
any bus-width host_caps in the driver and depend on the DT or
platform_data to have this info.

With drivers split 50/50, which is the right way?  Don't have the driver
report what bits it supports and depend on the device tree to have that
information?  Or have of_mci_parse remove widths that aren't indicated
as supported in the DT, so that the DT can remove widths the hardware
indicates it supports.