usb: musb: core: Fix handling of the phy notifications

We currently can't unload omap2430 MUSB platform glue driver module and
this cause issues for fixing the MUSB code further. The reason we can't
remove omap2430 is because it uses the PHY functions and also exports the
omap_musb_mailbox function that some PHY drivers are using.

Let's fix the issue by exporting a more generic musb_mailbox function
from the MUSB core and allow platform glue layers to register phy_callback
function as needed.

And now we can now also get rid of the include/linux/musb-omap.h.

Cc: Bin Liu <b-liu@ti.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: NeilBrown <neil@brown.name>
Reviewed-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>