clk: lochnagar: Update DT binding doc to include the primary SPDIF MCLK

This clock was missed when the binding was initially merged but is
supported by the driver, so add it to the binding document.
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/clock/cirrus,lochnagar.txt

@@ -40,6 +40,7 @@ Optional properties:
        input audio clocks from host system.
      - ln-psia1-mclk, ln-psia2-mclk : Optional input audio clocks from
        external connector.
+     - ln-spdif-mclk : Optional input audio clock from SPDIF.
      - ln-spdif-clkout : Optional input audio clock from SPDIF.
      - ln-adat-mclk : Optional input audio clock from ADAT.
      - ln-pmic-32k : On board fixed clock.