dt-bindings: rtc: mxc: Document fsl,imx31-rtc

imx31.dtsi uses the following RTC compatible:

compatible = "fsl,imx31-rtc", "fsl,imx21-rtc";

Document 'fsl,imx31-rtc' to fix the following dt-schema warning:

'fsl,imx31-rtc' is not one of ['fsl,imx1-rtc', 'fsl,imx21-rtc']

Signed-off-by: Fabio Estevam <festevam@denx.de>
Link: https://lore.kernel.org/r/20250112134027.1013213-1-festevam@gmail.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>