plat/arm: mhu: Move to drivers/ folder

Change-Id: I656753a1825ea7340a3708b950fa6b57455e9056
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/plat/arm/css/sgm/sgm_bl31_setup.c b/plat/arm/css/sgm/sgm_bl31_setup.c
index d9490b1..7e92ac8 100644
--- a/plat/arm/css/sgm/sgm_bl31_setup.c
+++ b/plat/arm/css/sgm/sgm_bl31_setup.c
@@ -6,11 +6,11 @@
 
 #include <common/bl_common.h>
 #include <common/debug.h>
+#include <drivers/arm/css/css_mhu_doorbell.h>
 #include <drivers/arm/css/scmi.h>
 #include <plat/arm/common/plat_arm.h>
 
 #include <sgm_plat_config.h>
-#include "../../css/drivers/mhu/css_mhu_doorbell.h"
 
 static scmi_channel_plat_info_t sgm775_scmi_plat_info = {
 		.scmi_mbx_mem = CSS_SCMI_PAYLOAD_BASE,