doc: DT: Add Generic Serial Device Tree Bindings

Document a set of generic properties for describing UARTs in a
device tree:
  1. The GPIO modem control properties are currently duplicated across
     hardware-specific binding documentation,
  2. The property for dedicated RTS/CTS hardware flow control lines is
     already supported by several drivers, albeit with a vendor-specific
     prefix, hence make it generic.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <rob@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>