dt-bindings: serial: imx: Document mandatory clock properties

The UART IP must be connected to clock, document the properties in DT bindings.
Update example to match Linux arch/arm/boot/dts/imx51.dtsi .

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230219142250.10176-2-marex@denx.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>