ARM: ux500: Enable support for RPMB and Reliable Write for eMMC

By adding MMC_CAP_CMD23 for the eMMC devices, we can support RPMB
and Reliable Write. Additionally it will mean CMD12 will not be sent
to end a successful data transfer.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>