usb: musb: Add a quirk to preserve the session during suspend

On da8xx, VBUS is not maintained during suspend when musb is in host mode.
On resume, all the connected devices will be disconnected and then will
be enumerated again.
This happens because MUSB_DEVCTL is cleared during suspend.
Add a quirk to not clear MUSB_DEVCTL and then preserve the  session during
a suspend.
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>

drivers/usb/musb/musb_core.c

@@ -2659,7 +2659,8 @@ static int musb_suspend(struct device *dev)
 
 	musb_platform_disable(musb);
 	musb_disable_interrupts(musb);
-	musb_writeb(musb->mregs, MUSB_DEVCTL, 0);
+	if (!(musb->io.quirks & MUSB_PRESERVE_SESSION))
+		musb_writeb(musb->mregs, MUSB_DEVCTL, 0);
 	WARN_ON(!list_empty(&musb->pending_list));
 
 	spin_lock_irqsave(&musb->lock, flags);

drivers/usb/musb/musb_core.h

@@ -172,6 +172,7 @@ struct musb_io;
  */
 struct musb_platform_ops {
 
+#define MUSB_PRESERVE_SESSION	BIT(7)
 #define MUSB_DMA_UX500		BIT(6)
 #define MUSB_DMA_CPPI41		BIT(5)
 #define MUSB_DMA_CPPI		BIT(4)