[U-Boot] [U-Boot,V4,1/2] mmc: add HS400 support

[Trent Piepho <tpiepho@impinj.com>]

I didn't see HS400 working on my IMX7d, even thought it appears it
should be supported.

The problem appears to be when this bit of code in fsl_esdhc.c, which
dates to a patch "mmc: fsl_esdhc: support SDR104 and HS200":

 static struct esdhc_soc_data usdhc_imx7d_data = {
         .flags = ESDHC_FLAG_USDHC | ESDHC_FLAG_STD_TUNING                                      
                         | ESDHC_FLAG_HAVE_CAP1 | ESDHC_FLAG_HS200
                         | ESDHC_FLAG_HS400,
         .caps = UHS_CAPS | MMC_MODE_HS200 | MMC_MODE_DDR_52MHz |
                 MMC_MODE_HS_52MHz | MMC_MODE_HS,
 };

Notice that MMC_MODE_HS400 isn't in the caps list.  Once I add that
uboot will attempt HS400 and it appears to work correctly.

I think maybe this patch should have added this to the caps?

Alternatively, there is a property that can be added to the device
tree, mmc-hs400-1_8v, that will add this cap.  But the code to parse
those dt properties, mmc_of_parse(), isn't used by the fsl_esdhc
driver, which has its own parsing code that doesn't know about mmc-
hs400-1_8v.

So maybe fsl-esdhc should use mmc_of_parse() and the appropriate iMX
DTs should add those properties?

I think the DT method might be better, since it would allow the DT to
not list a mode that doesn't work due to the the board.  I.e., the iMX
host supports the mode, the eMMC supports the mode, but the board
layout doesn't have traces that can support it.  Too long, not matched
well enough, etc.  It doesn't seem like there's an nice way to remove a
mode otherwise.