Documentation: gpio: Move GPIO mapping documentation to driver-api

Move gpio/board.txt to driver-api/gpio/board.rst and make sure it builds
cleanly as ReST.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/gpio/index.rst

@@ -10,6 +10,7 @@ Contents:
    intro
    driver
    consumer
+   board
    legacy
 
 Core