dt-bindings: document the MPS2 UART bindings

This adds documentation of device tree bindings for the
UART found on ARM MPS2 platform

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>