ARM: dts: stm32: Add coprocessor detach mbox on stm32mp15xx-osd32 SoM

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.

See 6257dfc1 for a patch that does the
same for stm32mp15x-dkx boards.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>