ARM: multi_v7_defconfig: Enable STM32 IPCC mailbox driver

The STM32 systems have a mailbox used for communication with non-Linux
processors like the M4 on the STM32MP157A.  Enable the driver for the
mailbox so it is available for testing.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240107-arm-defconfig-stm32-ipcc-v1-1-924721db5661@kernel.org


Signed-off-by: Arnd Bergmann <arnd@arndb.de>