omap mailbox: fix empty struct device for omap_mbox

Since "mbox->dev" doesn't exist and isn't created either at
registration, this patch will create "struct device", which belongs to
"omap-mailbox" class and set this pointer for the member of
"struct omap_mbox".

Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>