arm: dts: owl-s500: Add MMC support

Add MMC controller nodes for Actions Semi S500 SoC, in order to
facilitate access to SD/EMMC/SDIO cards.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>