spi: add SPI_MOSI_IDLE_LOW mode bit

Merge series from Boerge Struempfel <boerge.struempfel@gmail.com>:

Some spi controller switch the mosi line to high, whenever they are
idle. This may not be desired in all use cases. For example neopixel
leds can get confused and flicker due to misinterpreting the idle state.
Therefore, we introduce a new spi-mode bit, with which the idle behaviour
can be overwritten on a per device basis.