configs: stm32mp15: deactivate the CONFIG_STM32_IPCC

The IPCC mailbox is only used for communication with M4 firmware but
it is not used in the stm32 remoteproc driver; it was planed but the
support of this mailbox in remoteproc for STM32MP15x is dropped.

So the associated drivers and config CONFIG_STM32_IPCC can be
deactivated to reduce the U-Boot size; the CONFIG_DM_MAILBOX can be
also deactivated as the mailbox UCLASS is no more used.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>