mt8173 IC spi HW has 4 gpio group, it's possible to support
max <= 4 slave devices, even mtk spi HW is not congruent to spi core.
1. When a device do a spi_message transfer, spi HW should know
   which pad-group this device is on, and then writes pad-select
   register.
2. Mtk pad-select register just selects which MISO pin HW will
   receive data. For example, pad-select=1(select spi1 pins), HW just
   receives data from spi1 MISO, but it still send waveform to all 4
   group cs/clk/mosi. If cs pin in other groups is still spi mode,
   after spi1 is selected(by active cs pin), devices on other group
   will also be selected.
Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>