dt-bindings: aspeed: Add UART routing controller

Add dt-bindings for Aspeed UART routing controller.

Signed-off-by: Oskar Senft <osk@google.com>
Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210927023053.6728-4-chiawei_wang@aspeedtech.com