J721e uses a UART controller that is compatible with AM654 UART.
Introduce a specific compatible to help handle the differences if
necessary.

Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Vignesh R <vigneshr@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tero Kristo <t-kristo@ti.com>