mmc: sdhci-tegra: Add 8-bit support to device tree binding.

The previous patch which implemented a DT binding for sdhci-tegra did not
allow all platform data fields to be initialized from DT. The following
were missing:

is_8bit: Implemented by this patch.

pm_flags: Not implemented yet. There are no mainline users of this field.
  I'm not quite sure what it's for, and hence how to represent this
  in DT; the value ends up being assigned to host->mmc->pm_caps.

While we're at it, fix the binding documentation to refer to "SD/MMC"
instead of "eSDHC", since that's the correct name; "eSDHC" was cut/paste
from the Freescale binding docs.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>