mmc: Add Actions Semi Owl SoCs SD/MMC driver

Add SD/MMC driver for Actions Semi Owl SoCs. This driver currently
supports standard, high speed, SDR12, SDR25 and SDR50. DDR50 mode is
supported but it is untested. There is no SDIO support for now.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>