arm: dts: owl-s500-roseapplepi: Add uSD support

Add uSD support for RoseapplePi SBC using a fixed regulator as a
temporary solution until PMIC support becomes available.

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