serial: sirf: add DT-binding document to describle detailed properties

we defined some special properties for hardware flow control, document them
for DT-binding.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>