usb: musb: Add support for optional VBUS irq to dsps glue layer

We can now configure the PMIC interrupt to provide us VBUS
events. In that case we don't need to constantly poll the
status and can make it optional. This is only wired up
for the mini-B interface on beaglebone.

Note that eventually we should get also the connect status
for the host interface when the am335x internal PM coprocessor
provides us with an IRQ chip. For now, we still need to poll
for the host mode status.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>