refactor(tc): drop the 'mhu-protocol' property in DT binding

As the 'mhu-protocol' property is not used in mhu node, drop it.

Change-Id: I2f7320f668451ce44601dfa48bf47103334c39ed
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Signed-off-by: Leo Yan <leo.yan@arm.com>
diff --git a/fdts/tc-base.dtsi b/fdts/tc-base.dtsi
index 79f2666..e416bca 100644
--- a/fdts/tc-base.dtsi
+++ b/fdts/tc-base.dtsi
@@ -292,7 +292,6 @@
 		#mbox-cells = <2>;
 		interrupts = <GIC_SPI INT_MBOX_RX IRQ_TYPE_LEVEL_HIGH>;
 		interrupt-names = "mhu_rx";
-		mhu-protocol = "doorbell";
 		arm,mhuv2-protocols = <0 1>;
 	};
 
@@ -303,7 +302,6 @@
 		clock-names = "apb_pclk";
 		#mbox-cells = <2>;
 		interrupt-names = "mhu_tx";
-		mhu-protocol = "doorbell";
 		arm,mhuv2-protocols = <0 1>;
 	};