doc: dt: document altera-passive-serial binding

Describe an altera-passive-serial devicetree entry, required features

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>