arm64: dts: ti: k3-j7200: Enable UART nodes at the board level

UART nodes defined in the top-level J7200 SoC dtsi files are incomplete
and may not be functional unless they are extended with pinmux
information.

As the pinmux is only known at the board integration level, these
nodes should only be enabled when provided with this information.

Disable the UART nodes in the dtsi files and only enable the ones that
are actually pinned out on a given board.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>
Link: https://lore.kernel.org/r/20221020160305.18711-6-afd@ti.com