board: theobroma-systems: jaguar_rk3588: explicit UART IOMUX

board: theobroma-systems: jaguar_rk3588: explicit UART IOMUX

When one attempts to build Tiger and Jaguar from the same directory,
because Tiger will set the IOMUX to 2 but Jaguar doesn't reset it,
Jaguar will have IOMUX set to 2 if built after Tiger.

This isn't an issue if in separate directories or building Tiger after
Jaguar because the default if missing from the configuration file is 0.

Let's make it explicit that the iomux is 0 for Jaguar.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>