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/tools/Makefile b/tools/Makefile
index 6339f6a..f41e7c6 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -13,6 +13,7 @@
 	@echo '  cpupower               - a tool for all things x86 CPU power'
 	@echo '  firewire               - the userspace part of nosy, an IEEE-1394 traffic sniffer'
 	@echo '  freefall               - laptop accelerometer program for disk protection'
+	@echo '  gpio                   - GPIO tools'
 	@echo '  hv                     - tools used when in Hyper-V clients'
 	@echo '  iio                    - IIO tools'
 	@echo '  lguest                 - a minimal 32-bit x86 hypervisor'
@@ -53,7 +54,7 @@
 cpupower: FORCE
 	$(call descend,power/$@)
 
-cgroup firewire hv guest spi usb virtio vm net iio: FORCE
+cgroup firewire hv guest spi usb virtio vm net iio gpio: FORCE
 	$(call descend,$@)
 
 liblockdep: FORCE
@@ -119,7 +120,7 @@
 cpupower_clean:
 	$(call descend,power/cpupower,clean)
 
-cgroup_clean hv_clean firewire_clean lguest_clean spi_clean usb_clean virtio_clean vm_clean net_clean iio_clean:
+cgroup_clean hv_clean firewire_clean lguest_clean spi_clean usb_clean virtio_clean vm_clean net_clean iio_clean gpio_clean:
 	$(call descend,$(@:_clean=),clean)
 
 liblockdep_clean:
@@ -155,6 +156,7 @@
 clean: acpi_clean cgroup_clean cpupower_clean hv_clean firewire_clean lguest_clean \
 		perf_clean selftests_clean turbostat_clean spi_clean usb_clean virtio_clean \
 		vm_clean net_clean iio_clean x86_energy_perf_policy_clean tmon_clean \
-		freefall_clean build_clean libbpf_clean libsubcmd_clean liblockdep_clean
+		freefall_clean build_clean libbpf_clean libsubcmd_clean liblockdep_clean \
+		gpio_clean
 
 .PHONY: FORCE