Add Mediatek SPI driver

[leilk.liu at mediatek.com]

Mediatek SPI BUS controller has 3 hardware restrictions:
1. Hw has the restriction that in one transfer, length must be a multiple of
   1024, when it's greater than 1024bytes.
2. Hw tx/rx have 4bytes aligned restriction.
3. For MT8173 IC: RX must enable TX, then TX transfer dummy data; TX don't need
   to enable RX.
Some workarounds are done in SPI driver code base on v4.1-rc1.