ARM: OMAP4: add _dev_attr_ to ocp2scp for representing usb_phy

In order to reflect devices(usb_phy) attached to ocp2scp bus, ocp2scp
is assigned a device attribute to represent the attached devices.

This is needed to fix MUSB regression caused by commit c9e4412a
(arm: omap: phy: remove unused functions from omap-phy-internal.c)

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Benoit Cousson <b-cousson@ti.com>
[tony@atomide.com: updated comments for regression info]
Signed-off-by: Tony Lindgren <tony@atomide.com>