Added dedicated constants file for register offset values.
 On branch master
 Changes to be committed:

	modified:   CMakeLists.txt
	modified:   Doxyfile
	new file:   gpioconstants.h

 Changes not staged for commit:

	modified:   gpiodrive.c
	modified:   gpiodrive.h
Added fields to the GPIO_CONTEXT struct for direct register access.
 On branch master
 Changes to be committed:
	modified:   gpiodrive.c
	modified:   gpiodrive.h