GPIO via mmap support status

[KostyaEsmukov]

In the libraries there're two ways of GPIO usage: using sys files (implemented here) and with memory mapping. This issue is regarding the status of /dev/mem / /dev/gpiomem method support.

Libraries:

Name	Target Language	Board	/sys files	/dev/mem	Interrupts	Notes
bcm2835	C	RPi	❌	✅	❌
wiringPi	C	RPi	✅	✅	✅ 1
RPi.GPIO	Python	RPi	❓	✅ 2	✅ 3	It looks like /sys files are used for interrupts only. Needs further research 3
gpiozero	Python	RPi	✅	✅	✅	There're two strategies in the code: GPIOFS and GPIOMemory 4. By default the GPIOMemory one is used 5.
onoff	NodeJS	RPi, C.H.I.P., BeagleBone	✅	❌	✅
rpi-gpio.js	NodeJS	RPi	✅	❌	✅	Interrupts implemented using epoll 6

TODO:

• explain how can we implement supporting the /dev/mem method
• highlight the importance of /dev/mem method support

[flongo82]

Very nice job! By the way I would keep the use of /dev/mem with low priority.