ARM: dts: stm32: Add coprocessor detach mbox on stm32mp15x-dkx boards

To support the detach feature, add a new mailbox channel to inform
the remote processor on a detach. This signal allows the remote processor
firmware to stop IPC communication and to reinitialize the resources for
a re-attach.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>