driver - periph: Extend SPI low-level driver interface

This PR adds a new function to the low-level SPI driver interface and adapts the existing implementations accordingly. The new function is used to configure the SPI pins (CLK,MISO,MOSI) independently from the SPI initialization function.

The possibility to independently reconfigure the pins is especially useful if a pin shares SPI and GPIO functionality (e.g. MISO/GDO1 in CC110X) and switching between them is needed.
You added `SPI_2` here but no configuration in periph_conf.h?