ASoC: dt-bindings: fsl-sai: Add two PLL clock source

Add two PLL clock source, they are the parent clocks of root clock
one is for 8kHz series rates, another one is for 11kHz series rates.
They are optional clocks, if there are such clocks, then driver
can switch between them for supporting more accurate rates.
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1656667961-1799-7-git-send-email-shengjiu.wang@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/fsl-sai.txt

@@ -21,6 +21,9 @@ Required properties:
   - clock-names		: Must include the "bus" for register access and
 			  "mclk1", "mclk2", "mclk3" for bit clock and frame
 			  clock providing.
+                          "pll8k", "pll11k" are optional, they are the clock
+                          source for root clock, one is for 8kHz series rates
+                          another one is for 11kHz series rates.
   - dmas		: Generic dma devicetree binding as described in
 			  Documentation/devicetree/bindings/dma/dma.txt.