tools/gpio: create GPIO tools

This creates GPIO tools under tools/gpio/* and adds a single
example program to list the GPIOs on a system. When proper
devices are created it provides this minimal output:

Cc: Johan Hovold <johan@kernel.org>
Cc: Michael Welling <mwelling@ieee.org>
Cc: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 76986c3..4d3d1b8 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4820,6 +4820,7 @@
 F:	include/linux/gpio.h
 F:	include/asm-generic/gpio.h
 F:	include/uapi/linux/gpio.h
+F:	tools/gpio/
 
 GRE DEMULTIPLEXER DRIVER
 M:	Dmitry Kozlov <xeb@mail.ru>