ARM: dts: OMAP4: Fix broken pbias device creation

commit <7415b0b4> ("ARM: dts: omap4: 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
omap4_padconf_global. After this device for pbias_regulator
is not created.

Fix it by adding "simple-bus" compatible property to
omap4_padconf_global dt node.

Fixes: 7415b0b4 ("ARM: dts: omap4: add minimal l4 bus layout
with control module support")

Cc: <stable@vger.kernel.org> # v4.1
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>