[REPOST PATCHv3] ARM: dts: OMAP2+: Add sub mailboxes device node information

* Suman Anna [off-list ref] [140910 12:28]:

The sub-mailbox devices are added to the Mailbox DT nodes on
OMAP2420, OMAP2430, OMAP3, AM33xx, AM43xx, OMAP4 and OMAP5
family of SoCs. This data represents the same mailboxes that
used to be represented in hwmod attribute data previously.
The node name is chosen based on the .name field of
omap_mbox_dev_info structure used in the hwmod data.

Thanks applying into omap-for-v3.18/dt.

Tony