dt-bindings: mailbox: fsl,mu: add new tx doorbell channel

Add new tx doorbell channel for i.MX95 SCMI mailbox usage.
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/fsl,mu.yaml

@@ -72,9 +72,9 @@ properties:
       type      : Channel type
       channel   : Channel number
 
-      This MU support 5 type of unidirectional channels, each type
+      This MU support 6 type of unidirectional channels, each type
       has 4 channels except RST channel which only has 1 channel.
-      A total of 17 channels.  Following types are
+      A total of 21 channels.  Following types are
       supported:
       0 - TX channel with 32bit transmit register and IRQ transmit
           acknowledgment support.
@@ -82,6 +82,7 @@ properties:
       2 - TX doorbell channel. Without own register and no ACK support.
       3 - RX doorbell channel.
       4 - RST channel
+      5 - Tx doorbell channel. With S/W ACK from the other side.
     const: 2
 
   clocks: