Update amba_driver.remove to comply with v5.12

As of kernel version v5.12 the amba_driver.remove is expected to be of
'static void' rather than the previous 'static int'. This patch updates
the mhu files to comply with kernel version v5.12.

Change-Id: Iec4d7927e0db2c8008049b743ca890647a2fa9ff
diff --git a/mailbox/arm_mhu_v2.c b/mailbox/arm_mhu_v2.c
index 66ce216..b65bd78 100644
--- a/mailbox/arm_mhu_v2.c
+++ b/mailbox/arm_mhu_v2.c
@@ -18,6 +18,7 @@
 #include <linux/mailbox_controller.h>
 #include <linux/of_device.h>
 #include <linux/of_address.h>
+#include <linux/version.h>
 
 #define MHU_V2_REG_STAT_OFS             0x0
 #define MHU_V2_REG_CLR_OFS              0x8
@@ -264,6 +265,14 @@
 	return 0;
 }
 
+#if KERNEL_VERSION(5, 12, 0) <= LINUX_VERSION_CODE
+static void mhuv2_remove(struct amba_device *adev)
+{
+	struct arm_mhuv2 *mhuv2 = amba_get_drvdata(adev);
+
+	mbox_controller_unregister(&mhuv2->mbox);
+}
+#else
 static int mhuv2_remove(struct amba_device *adev)
 {
 	struct arm_mhuv2 *mhuv2 = amba_get_drvdata(adev);
@@ -272,6 +281,8 @@
 
 	return 0;
 }
+#endif
+
 
 static struct amba_id mhuv2_ids[] = {
 	{