arm64: dts: imx8mp-var-som: Move PCA9534 GPIO expander to Symphony carrier

arm64: dts: imx8mp-var-som: Move PCA9534 GPIO expander to Symphony carrier

The VAR-SOM-MX8MP module does not include the PCA9534 GPIO expander nor
the LED connected to it. These components are present only on the
Symphony carrier board and may vary across custom carrier designs.

Move the PCA9534 GPIO expander node and the associated LED definition
from the SOM device tree to the Symphony carrier DTS, ensuring the SOM
dtsi describes only hardware present on the module.

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