OMAP: mailbox: build device using omap_device/omap_hwmod

Remove static platform_device and resource data within
omap mailbox driver; use the one defined in the hwmod
database along with omap_device framework for device
build and registration.

Add device latency functions to be used, so clock can be
enabled and sysconfig is configured.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Omar Ramirez Luna <omar.ramirez@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>