[NEW] SPI driver improvements

[Giovanni]

Hi,

The SPI driver model received some improvements, the CS selection can be done in 5 different ways:

0) None, spiSelect() and spiUnselect() are do nothing, this is useful when using some HW CS handling, no space wasted in the configuration structure.
1) Pad, it is the current mode, you specify port and pad.
2) Port, you specify port and bit mask, faster than 1) because it saves a bit shift, on CM0 it saves 3 instruction in select and 3 in unselect, fastest mode when latency is important.
3) Line, it uses an ioline_t for selection, saves space in the configuration structure but it is a bit slower.
4) LLD-defined, it does not use PAL at all, implementation is delegated to the LLD, not used on STM32.

Now PAL-related fields are no more part of the variant part of the configuration structure, those are a standard feature.

Note for the maintainers, the default is the classic mode (pad) but all other implementations should be aligned to the STM32 one.

Giovanni