ARM: OMAP2: MUSB: Specify omap4 has mailbox

Added has_mailbox to the musb platform data to specify that omap uses
an external mailbox (in control module) to communicate with the musb
core during device connect and disconnect.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>