dt-bindings: serial: 8250: Add aspeed,sirq-polarity-sense.

Add documentation for 8250_aspeed_vuart's aspeed,sirq-polarity-sense
property that enables to auto-configure the VUART's SIRQ polarity.

Signed-off-by: Oskar Senft <osk@google.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20190905144130.220713-2-osk@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>