pwm: 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>
mmap: remove implementation of mmap gpio

Instead of trying to have an implementation that covers most platforms
Leave all mmap up to platform definition. Through function pointers.

gpio_mmap_setup sets up the mmap
gpio_mmap_write gpio write

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