arm64: dts: imx8mp-var-som: Add support for WM8904 audio codec

arm64: dts: imx8mp-var-som: Add support for WM8904 audio codec

The VAR-SOM-MX8MP can integrate the WM8904, a high-performance
ultra-low-power stereo codec optimized for portable audio applications.

This patch adds the WM8904 device to the appropriate I2C bus, enables
the SAI peripheral, and introduces the sound node to expose the
sound card to the system.

Signed-off-by: Stefano Radaelli <stefano.r@variscite.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>