dt-bindings: mmc: owl: Add compatible string for Actions Semi S500 SoC

Add a new compatible string corresponding to the MMC controller found
in the S500 variant of the Actions Semi Owl SoCs family.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Link: https://lore.kernel.org/r/713f57f4a66fff9a50a8d93643a7987d6560becb.1605823502.git.cristian.ciocaltea@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>