Documentation: DT: Add bindings for FSL NS16550A UART

This patch addss the device-tree documentation for Freescale's
NS16550 UART (also called DUART).

There is a specific errata fix required in FSL NS16550 UART
which ensures that an random interrupt storm is not observed when
a break is provided as an input to the UART.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>