arm64: dts: freescale: imx8mm-var-som: Move UART4 description to Symphony

arm64: dts: freescale: imx8mm-var-som: Move UART4 description to Symphony

The VAR-SOM-MX8MM module does not provide an onboard debug console.
UART4 is routed and exposed only on the Symphony carrier board, while
custom carrier designs may choose to expose a different UART.

Move the UART4 node from the SOM device tree to the
imx8mm-var-som-symphony.dts, keeping the SOM dtsi limited to hardware
present on the module itself.

Signed-off-by: Stefano Radaelli <stefano.r@variscite.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>