mmc: sdhci: Add sdhci_adma_mark_end()

In preparation for 64-bit ADMA, separate out code
that touches the ADMA descriptor by adding
sdhci_adma_mark_end().

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