mmc: sdhci_am654: Add Support for 8 bit IP on J721E

The 8 bit IP on the TI's J721E device departs from the AM654x IP in some
ways which require special handling. Create a driver_data structure
which holds the pltfm_data and a flags field which is used to indicate
these differences. These are the following:

1. The pins are not muxed with anything else inside the SoC and hence the
   IOMUX_ENABLE field does not exist. Add a flag which is used to
   indicate the presence of the field.

2. The register field used to select DLL frequency is 3 bit wide as
   compared to 2 bits in AM65x. Add another flag which differentiates
   between 3 bit and 2 bit fields.

3. The strobe select field is 8 bit wide as compared to 4 bits for
   AM65x. Add yet another flag to indicate this difference. Strobe select
   is used only for HS400 speed mode, support for which has not yet been
   added in AM65x.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>