dt-bindings: serial: Document Actions Semi Owl UARTs

This UART is found on S500 and S900 SoCs.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>