dt-bindings: mailbox: imx-mu: add RST channel

i.MX MU has a MUR bit which is to reset both the Processor B and the
Processor A sides of the MU module, forcing all control and status
registers to return to their default values (except the BHR bit in the ACR
register and BHRM bit in BCR register), and all internal states to be
cleared.

Signed-off-by: Peng Fan <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

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

···
       type      : Channel type
       channel   : Channel number
 
-      This MU support 4 type of unidirectional channels, each type
-      has 4 channels. A total of 16 channels. Following types are
+      This MU support 5 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
       supported:
       0 - TX channel with 32bit transmit register and IRQ transmit
           acknowledgment support.
       1 - RX channel with 32bit receive register and IRQ support
       2 - TX doorbell channel. Without own register and no ACK support.
       3 - RX doorbell channel.
+      4 - RST channel
     const: 2
 
   clocks: