dt-bindings: PCI: qcom: Document ddrss_sf_tbu clock for sm8250

On SM8250 additional clock is required for PCIe devices to access NOC.
Document this requirement in devicetree bindings.

Link: https://lore.kernel.org/r/20210117013114.441973-2-dmitry.baryshkov@linaro.org
Fixes: 45816824 ("dt-bindings: pci: qcom: Document PCIe bindings for SM8250 SoC")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

Documentation/devicetree/bindings/pci/qcom,pcie.txt

@@ -132,8 +132,20 @@
 			- "master_bus"	AXI Master clock
 			- "slave_bus"	AXI Slave clock
 
--clock-names:
-	Usage: required for sdm845 and sm8250
+- clock-names:
+	Usage: required for sdm845
+	Value type: <stringlist>
+	Definition: Should contain the following entries
+			- "aux"		Auxiliary clock
+			- "cfg"		Configuration clock
+			- "bus_master"	Master AXI clock
+			- "bus_slave"	Slave AXI clock
+			- "slave_q2a"	Slave Q2A clock
+			- "tbu"		PCIe TBU clock
+			- "pipe"	PIPE clock
+
+- clock-names:
+	Usage: required for sm8250
 	Value type: <stringlist>
 	Definition: Should contain the following entries
 			- "aux"		Auxiliary clock
@@ -142,6 +154,7 @@
 			- "bus_slave"	Slave AXI clock
 			- "slave_q2a"	Slave Q2A clock
 			- "tbu"		PCIe TBU clock
+			- "ddrss_sf_tbu" PCIe SF TBU clock
 			- "pipe"	PIPE clock
 
 - resets: