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

Documentation/devicetree/bindings/serial/8250.txt

@@ -56,6 +56,11 @@ Optional properties:
 - {rts,cts,dtr,dsr,rng,dcd}-gpios: specify a GPIO for RTS/CTS/DTR/DSR/RI/DCD
   line respectively. It will use specified GPIO instead of the peripheral
   function pin for the UART feature. If unsure, don't specify this property.
+- aspeed,sirq-polarity-sense: Only applicable to aspeed,ast2500-vuart.
+  phandle to aspeed,ast2500-scu compatible syscon alongside register offset
+  and bit number to identify how the SIRQ polarity should be configured.
+  One possible data source is the LPC/eSPI mode bit.
+  Example: aspeed,sirq-polarity-sense = <&syscon 0x70 25>
 
 Note:
 * fsl,ns16550: