mailbox: stm32-ipcc: add COMPILE_TEST dependency

This allows compiling the driver on architectures where the hardware is not
available. Most other mailbox drivers support this as well.
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/Kconfig

@@ -201,7 +201,7 @@ config BCM_FLEXRM_MBOX
 
 config STM32_IPCC
 	tristate "STM32 IPCC Mailbox"
-	depends on MACH_STM32MP157
+	depends on MACH_STM32MP157 || COMPILE_TEST
 	help
 	  Mailbox implementation for STMicroelectonics STM32 family chips
 	  with hardware for Inter-Processor Communication Controller (IPCC)