usb: dwc3: omap: Add an API to write to dwc mailbox

Add an API in the omap glue layer to write to the mailbox register which
can be used by comparator driver(twl). To pass the detection of the attached
device (signified by VBUS, ID) to the dwc3 core, dwc3 core has to write
to the mailbox regiter.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>