ARM: OMAP4: USB: power down MUSB PHY during boot

Commit c9e4412ab8eb8ef82d645d8749c4ce96ad490007 removed all of the USB
PHY functions for OMAP4, but this causes a problem with core retention
as the MUSB module remains enabled if omap-usb2 phy driver is not used.
This keeps the USB DPLL enabled and prevents l3_init pwrdm from idling.

Fixed by adding a minimal function back that disables the USB PHY during
boot.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>