arm64: dts: ti: am65/j721e/j7200: Mark firmware used uart as "reserved"

Follow the device tree standards that states to set the
status="reserved" if an device is operational, but used by a non-linux
firmware in the system.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20201113211826.13087-6-nm@ti.com