arm64: dts: imx8mp-var-som: Move USDHC2 support to Symphony carrier

arm64: dts: imx8mp-var-som: Move USDHC2 support to Symphony carrier

The VAR-SOM-MX8MP module does not include a microSD slot connected to
USDHC2. The USDHC2 interface is routed only on the Symphony carrier
board, and it may optionally be used or omitted depending on the
customer's carrier design.

Move the USDHC2 node, its regulators, pinctrl groups and related GPIOs
from the SOM device tree to the Symphony carrier DTS, keeping the SOM
description limited to hardware populated on the module.

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