ARM: dts: OMAP2+: Replace underscores in sub-mailbox node names

ARM: dts: OMAP2+: Replace underscores in sub-mailbox node names

A number of sub-mailbox node names in various OMAP2+ dts files are
currently using underscores. This is not adhering to the node name
convention, fix all of these to use hiphens.

These nodes are already using the prefix mbox, so they will be in
compliance with the sub-mailbox node name convention being added in
the OMAP Mailbox YAML binding as well.

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