arm64: dts: imx8mp-var-som: Move UART2 description to Symphony carrier

arm64: dts: imx8mp-var-som: Move UART2 description to Symphony carrier

The VAR-SOM-MX8MP module does not provide an onboard debug console.
UART2 is routed and exposed only on the Symphony carrier board, while
custom carrier designs may choose to expose a different UART.

Move the UART2 node from the SOM device tree to the
imx8mp-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: Shawn Guo <shawnguo@kernel.org>