tty: ar933x_uart: move devicetree binding documentation

Commit 'tty: ar933x_uart: add device tree support
and binding documentation' introduced a new doc in
bindins/tty/serial.

According to a recent thread [1] on the linux-serial
list, the binding documentation of serial drivers
should be added into bindings/serial.

Move the documentation of qca,ar9330-uart to the
correct place.

  1. http://marc.info/?l=linux-serial&m=137771295411517

Cc: devicetree@vger.kernel.org
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>