spi: move spi setup from core into module

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>
pmw: move pwm setup to module

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>
internal: remove un-implmented function prototype

Left over from earlier version of mraa.

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>