Changed in v4:
- Move dma properties from dtsi to dts.
- Cancle the macro(SERIAL_FSL_LPUART_DMA)  .
- Separate the document for clocks which undocumented before into a single patch.
- Change some explanations in document(clocks, clock-names, dmas, dma-names).
- Change "lpuart-tx" and "lpuart-rx" to "tx" and "rx".

Changed in v3:
- Use the streaming DMA API for receive.
- Add the macro(SERIAL_FSL_LPUART_DMA) and dts node propertie for whether using the dma.
- Adjust some coding style.
 
Changed in v2:
- Add eDMA support for lpuart receive.
- Use dma_mapping_error test dma_map_single.
- Change some names of variable.
- Fix some bugs.
 
Added in v1:
- Add device tree bindings for lupart eDMA support.
- Add eDMA support for lpuart send.