dt-bindings: create arm/nxp folder and move LPC32xx SoC description to it

Create a separate folder for device tree bindings of NXP SoCs devices,
and move lpc32xx.txt to it.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NVladimir Zapolskiy <vz@mleia.com>