gpio: split gpiod board registration into machine header
As per example from the regulator subsystem: put all defines and
functions related to registering board info for GPIO descriptors
into a separate <linux/gpio/machine.h> header.
Cc: Andrew Victor <linux@maxim.org.za> Cc: Nicolas Ferre <nicolas.ferre@atmel.com> Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: Thierry Reding <thierry.reding@gmail.com> Acked-by: Stephen Warren <swarren@wwwdotorg.org> Reviewed-by: Alexandre Courbot <gnurou@gmail.com> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>