mailbox/omap: add a parent structure for every IP instance

A new structure, omap_mbox_device, is added to contain
the global variables pertinent to a mailbox h/w IP block.
This enables the support for having multiple instances of
the same h/w IP block in the SoC. This is in preparation
to support the DRA7 SoC, which is the first SoC in the OMAP
family to have multiple mailbox IP instances.

The changes include enhancements to the sub-mailbox registration
logic and mbox startup sequencing, removing the usage of single
global configuration variables for all h/w instances, and storing
the registered sub-mailboxes with the parent mailbox device
structure.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>