dt-bindings: serial: lpuart: add the clock requirement for imx8qxp

Add the baud clock requirement for imx8qxp.

Signed-off-by: Fugang Duan <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

Documentation/devicetree/bindings/serial/fsl-lpuart.txt

···
 - reg : Address and length of the register set for the device
 - interrupts : Should contain uart interrupt
 - clocks : phandle + clock specifier pairs, one for each entry in clock-names
-- clock-names : should contain: "ipg" - the uart clock
+- clock-names : For vf610/ls1021a/imx7ulp, "ipg" clock is for uart bus/baud
+  clock. For imx8qxp lpuart, "ipg" clock is bus clock that is used to access
+  lpuart controller registers, it also requires "baud" clock for module to
+  receive/transmit data.
 
 Optional properties:
 - dmas: A list of two dma specifiers, one for each entry in dma-names.