mmc: sdhci_am654: Add Support for 4 bit IP on J721E

Add support for 4 bit instances on TI's J721E devices. Because these
instances have no DLL, introduce a DLL_PRESENT flag and make sure DLL
related registers are only accessed when it is present. Also add a
separate set_clock callback for this compatible.

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>