fix(scmi): induce a delay in monitoring SCMI channel status

Reading the SCMI mailbox status in polling mode causes a burst of bus
accesses. On certain platforms, this would not be ideal as the shared
bus on the CPU subsystem might cause contentions across all the CPUs.
So allow platforms to specify a delay to be introduced while polling.

Change-Id: Ib90ad7b5954854071cfd543f4a27a178dde3d5c6
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
diff --git a/include/drivers/arm/css/scmi.h b/include/drivers/arm/css/scmi.h
index 356012b..96e1924 100644
--- a/include/drivers/arm/css/scmi.h
+++ b/include/drivers/arm/css/scmi.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2021, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2024, Arm Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -123,6 +123,8 @@
 	void (*ring_doorbell)(struct scmi_channel_plat_info *plat_info);
 	/* cookie is unused now. But added for future enhancements. */
 	void *cookie;
+	/* Delay in micro-seconds while polling the channel status. */
+	uint32_t delay;
 } scmi_channel_plat_info_t;