usb: musb: da8xx: Add support of suspend / resume

Implement PM methods specifics for da8xx glue.
The only thing to do is to power off the phy.
As the registers are in retention during suspend,
there is no need to save them.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>