firmware: arm_scmi: Fix the build when CONFIG_MAILBOX is not selected

0day CI kernel test robot reported following build error with randconfig

aarch64-linux-ld: drivers/firmware/arm_scmi/driver.o:(.rodata+0x1e0):
		undefined reference to `scmi_mailbox_desc'

Fix the error by adding CONFIG_MAILBOX dependency for scmi_mailbox_desc.

Link: https://lore.kernel.org/r/20210603072631.1660963-1-sudeep.holla@arm.com
Cc: Etienne Carriere <etienne.carriere@linaro.org>
Cc: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/driver.c

@@ -1571,7 +1571,9 @@ ATTRIBUTE_GROUPS(versions);
 
 /* Each compatible listed below must have descriptor associated with it */
 static const struct of_device_id scmi_of_match[] = {
+#ifdef CONFIG_MAILBOX
 	{ .compatible = "arm,scmi", .data = &scmi_mailbox_desc },
+#endif
 #ifdef CONFIG_HAVE_ARM_SMCCC_DISCOVERY
 	{ .compatible = "arm,scmi-smc", .data = &scmi_smc_desc},
 #endif