schemas: pci: bridge: Document PCIe N_FTS

Per PCIe r6.0, sec 4.2.5.1, during Link training, a PCIe component
captures the N_FTS value it receives.  Per 4.2.5.6, when
transitioning the Link from L0s to L0, it must transmit N_FTS Fast
Training Sequences to enable the receiver to obtain bit and Symbol
lock.

Components may have device-specific ways to configure N_FTS values
to advertise during Link training.  Define an n_fts array with an
entry for each supported data rate.

Signed-off-by: Krishna Chaitanya Chundru <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Rob Herring (Arm) <[email protected]>

Author: krishnachaitanya-linux

dtschema/schemas/pci/pci-bus-common.yaml

@@ -128,6 +128,16 @@ properties:
     $ref: /schemas/types.yaml#/definitions/uint32
     enum: [ 1, 2, 4, 8, 16, 32 ]
 
+  n-fts:
+    description:
+      The number of Fast Training Sequences (N_FTS) required by the
+      Receiver (this component) when transitioning the Link from L0s
+      to L0; advertised during initial Link training. Each entry in
+      the array specifies a PCIe data rate
+    $ref: /schemas/types.yaml#/definitions/uint8-array
+    minItems: 1
+    maxItems: 5
+
   reset-gpios:
     description: GPIO controlled connection to PERST# signal
     maxItems: 1