boards: Add support for Waveshare BLE400 board.

Added support for Waveshare BLE400 board.

Tested it with the following samples and confirm it work.

 - samples/basic/blinky
 - samples/basic/button
 - samples/basic/fade_led
 - samples/basic/threads

Signed-off-by: Roman Tataurov <diytronic@yandex.ru>