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/sgi/sgi_bl31_setup.c b/plat/arm/css/sgi/sgi_bl31_setup.c
index 82d8fed..bfcb521 100644
--- a/plat/arm/css/sgi/sgi_bl31_setup.c
+++ b/plat/arm/css/sgi/sgi_bl31_setup.c
@@ -10,14 +10,13 @@
 
 #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 <sgi_ras.h>
 #include <sgi_variant.h>
 
-#include "../../css/drivers/mhu/css_mhu_doorbell.h"
-
 sgi_platform_info_t sgi_plat_info;
 
 static scmi_channel_plat_info_t sgi575_scmi_plat_info = {