Convert binding doc ltc2978.txt to yaml format.
Additional change:
- add i2c node.

Fix below warning:
arch/arm64/boot/dts/freescale/fsl-lx2160a-clearfog-cx.dtb: /soc/i2c@2000000/i2c-mux@77/i2c@2/regulator@5c:
	failed to match any schema with compatible: ['lltc,ltc3882']
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Message-ID: <20240829150641.1307906-1-Frank.Li@nxp.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>