ARM: DTS: OMAP: add mailbox nodes for OMAP2+

The mailbox DT node data has been added for OMAP2420,
OMAP2430, OMAP3430/OMAP3630, OMAP44xx and AM33xx devices.
Data for OMAP5 is skipped for now since the corresponding
hwmod entry is not present.

The mailbox static device initialization logic is also
adjusted for a DT boot.

TODO:
- Revise this to use interrupt macros from Florian's
  series.
- Add dependent patches for OMAP5 & AM437x if base support
  is available for them in kernel.

Signed-off-by: Suman Anna <s-anna@ti.com>