Add missing "detach" mailbox to this board to permit the CPU 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.

Without this mailbox, detach is not possible and kernel log contains the
following warning to, so make sure all the STM32MP15xx platform DTs are
in sync regarding the mailboxes to fix the detach issue and the warning:
"
stm32-rproc 10000000.m4: mbox_request_channel_byname() could not locate channel named "detach"
"

Fixes: 6257dfc1 ("ARM: dts: stm32: Add coprocessor detach mbox on stm32mp15x-dkx boards")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>