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/mhu/css_mhu.h b/plat/arm/css/drivers/mhu/css_mhu.h
new file mode 100644
index 0000000..0fb00c7
--- /dev/null
+++ b/plat/arm/css/drivers/mhu/css_mhu.h
@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2014-2018, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef __CSS_MHU_H__
+#define __CSS_MHU_H__
+
+#include <stdint.h>
+
+void mhu_secure_message_start(unsigned int slot_id);
+void mhu_secure_message_send(unsigned int slot_id);
+uint32_t mhu_secure_message_wait(void);
+void mhu_secure_message_end(unsigned int slot_id);
+
+void mhu_secure_init(void);
+
+#endif	/* __CSS_MHU_H__ */