[nrf fromlist] bindings: nrf21540: set default tx-en-settle-time-us to 26

The tx-en-settle-time-us is set to 26 to take into account
the time needed for the RF output power rise time of the nRF5 SoC.

Upstream PR #: 89465

Signed-off-by: Andrzej Kuros <[email protected]>
(cherry picked from commit 893c333)

Author: ankuns

dts/bindings/net/wireless/nordic,nrf21540-fem.yaml

@@ -95,13 +95,16 @@ properties:
         This must be present to support SPI control of the FEM.
   tx-en-settle-time-us:
     type: int
-    default: 11
+    default: 26
     description: |
         Settling time in microseconds from state PG to TX.
 
-        Default value is based on Table 6 of the nRF21540 Product
-        Specification (v1.0), and can be overridden for tuned
-        configurations.
+        Default value of 11 microseconds is based on Table 6 of the nRF21540
+        Product Specification (v1.0). However the FEM must be ready when
+        the RF power starts to rise at the output of the SoC. That's why
+        the default value is increased by the time needed by the RF power
+        to rise at the output of the nRF5 SoCs.
+        This time can be overridden for tuned configurations.
   rx-en-settle-time-us:
     type: int
     default: 11