Add a driver for the Cadence SD4HC SD/SDIO/eMMC Controller.

For SD, it basically relies on the SDHCI standard code.
For eMMC, this driver provides some callbacks to support the
hardware part that is specific to this IP design.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>