dts: bindings: serial: nrf-uarte: Add timer property

Timer property indicates which TIMER instance should be used for
byte counting. If timer property is present then given instance
is using TIMER to count received bytes.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

Author: nordic-krch

dts/bindings/serial/nordic,nrf-uarte.yaml

@@ -14,3 +14,11 @@ properties:
     type: boolean
     description: |
       UARTE has RX frame timeout HW feature.
+
+  timer:
+    type: phandle
+    description: |
+      Timer instance used to count received bytes. Due to issues with frame timeout
+      feature it is required to reliably receive data in cases where flow control
+      is not used and new byte can appear on the line when frame timeout expires
+      but before it is handled.