mmc: dw_mmc: hi3798cv200: add MMC_CAP_CMD23 cap

Enable access to the RPMB on the on-board eMMC of the
Poplar board.

Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>