dt-bindings: fsl: rcpm: Add 'little-endian' and update Chassis definition

By default, QorIQ SoC's RCPM register block is Big Endian. But
there are some exceptions, such as LS1088A and LS2088A, are
Little Endian. So add this optional property to help identify
them.

Actually LS2021A and other Layerscapes won't totally follow Chassis
2.1, so separate them from powerpc SoC.

Signed-off-by: Ran Wang <ran.wang_1@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Li Yang <leoyang.li@nxp.com>