ARM: dts: dra7: Fix broken pbias device creation

commit <d919501f> ("ARM: dts: dra7: add minimal l4 bus
layout with control module support") moved pbias_regulator dt node
from being a child node of ocp to be the child node of
scm_conf. After this device for pbias_regulator is
not created.

Fix it by adding "simple-bus" compatible property to
scm_conf dt node.

Fixes: d919501f ("ARM: dts: dra7: add minimal l4 bus
layout with control module support")

Cc: <stable@vger.kernel.org> # v4.1
Suggested-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Tested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>