mailbox/omap: add support for parsing dt devices

Logic has been added to the OMAP2+ mailbox code to parse the
mailbox dt nodes and construct the different sub-mailboxes
associated with the instance. The DT representation of the
sub-mailbox devices is different from legacy platform data
representation to allow flexibility of interrupt configuration
between Tx and Rx fifos (to also possibly allow simplex devices
in the future). The DT representation gathers similar information
that was being passed previously through the platform data, except
for the interrupt type information, which is gathered through driver
compatible match data.

The non-DT support has to be maintained for now to not break
OMAP3 legacy boot, and the legacy-style code will be cleaned
up once OMAP3 is also converted to DT-boot only.

Cc: Jassi Brar <jassisinghbrar@gmail.com>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Tony Lindgren <tony@atomide.com>