ARM: dts: OMAP2/OMAP3: Rename processor sub-mailbox nodes

ARM: dts: OMAP2/OMAP3: Rename processor sub-mailbox nodes

The OMAP sub-mailbox used to communicate with the DSP and IVA remote
processors are currently named after the processor name. These can be
confused with the remote processors themselves. Rename them to remove
the ambiguity and use the prefix mbox to also adhere to the sub-mailbox
node name convention being added in the OMAP Mailbox YAML binding.

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