plat/arm: Add MHUv2 support to SCMI driver

Currently the SCMI driver supports MHUv1, but Arm platforms may have
varied versions of MHU driver, with MHUv2 controllers being in the
latest Arm platforms.

This patch updates the SCMI driver to support MHUv2, specifically that
the sender must send the wake-up to the receiver before initiating any
data transfer.

Also, the existing mhu driver files, css_mhu.c and css_mhu.h, have been
moved from the scpi directory to a new directory, css/drivers/mhu.

Change-Id: I9b46b492a3e1d9e26db12d83a9773958a8c8402f
Signed-off-by: Samarth Parikh <samarth.parikh@arm.com>
diff --git a/plat/arm/css/drivers/scmi/scmi.h b/plat/arm/css/drivers/scmi/scmi.h
index 850402a..cf9ef5e 100644
--- a/plat/arm/css/drivers/scmi/scmi.h
+++ b/plat/arm/css/drivers/scmi/scmi.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2018, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -96,6 +96,10 @@
 	uint32_t db_preserve_mask;
 	/* The bit mask that need to be set to ring doorbell */
 	uint32_t db_modify_mask;
+	/* The handler for ringing doorbell */
+	void (*ring_doorbell)(struct scmi_channel_plat_info *plat_info);
+	/* cookie is unused now. But added for future enhancements. */
+	void *cookie;
 } scmi_channel_plat_info_t;
 
 /*