dra7: dtsi: mark ocp2scp bus compatible with "simple-bus"

This is needed to probe devices under that bus such as the SATA PHY.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>