ARM: dts: dra7-ipu-dsp-common: Move mailboxes into common files

ARM: dts: dra7-ipu-dsp-common: Move mailboxes into common files

The System Mailboxes 5 and 6 and their corresponding child sub-mailbox
(IPC 3.x) nodes are enabled in each of the DRA7xx and AM57xx board
dts files individually at present. These mailboxes enable the Remote
Processor Messaging (RPMsg) communication stack between the MPU host
processor and each of the IPU1, IPU2, DSP1 and DSP2 remote processors.

Move these nodes into two common dtsi files - dra7-ipu-dsp-common and
dra74-ipu-dsp-common files, which are then included in various board
dts files. These files can be used to add all the common configuration
properties (except memory data) required by remote processor nodes.
The memory pools and the remote processor nodes themselves are to be
enabled in the actual board dts files. The first file is to used by
platforms using DRA72x/DRA71x/AM571x/AM570x SoCs, and the second file
is to be used by platforms using DRA74x/DRA76x/AM572x/AM574x SoCs.
The second file includes the first file and contains additional data
only applicable for DSP2 remote processor.

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