[gpioths] new driver for temperature/humidity sensor DHT11

This patch adds driver for temperature/humidity sensor connected via GPIO.
To compile it into kernel add "device gpioths". To activate driver, use
hints (.at and .pins) for gpiobus. As result it will provide temperature &
humidity values via sysctl.

DHT11 is cheap & popular temperature/humidity sensor used via GPIO on ARM
or MIPS devices like Raspberry Pi or Onion Omega.

Reviewed by: adrian
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D9185