stm32mp15: deactivate CONFIG_SCMI_AGENT_MAILBOX

Deactivate the SCMI agent mailbox which is not used on STM32MP15
platforms.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
diff --git a/configs/stm32mp15_trusted_defconfig b/configs/stm32mp15_trusted_defconfig
index feca26e..270cbaa 100644
--- a/configs/stm32mp15_trusted_defconfig
+++ b/configs/stm32mp15_trusted_defconfig
@@ -78,6 +78,7 @@
 CONFIG_FASTBOOT_CMD_OEM_FORMAT=y
 CONFIG_FASTBOOT_CMD_OEM_PARTCONF=y
 CONFIG_FASTBOOT_CMD_OEM_BOOTBUS=y
+# CONFIG_SCMI_AGENT_MAILBOX is not set
 CONFIG_GPIO_HOG=y
 CONFIG_DM_HWSPINLOCK=y
 CONFIG_HWSPINLOCK_STM32=y