documentation: Add documentation on setting up mainboard GPIOs

The new documentation describes typical ways that mainboards will
set up their GPIOs, as well as the distinction between "early"
and "normal" GPIOs.  It also describes the typical properties
that GPIO configuration will cover.

Change-Id: I279eec4ed2bb0248a2bdb363fb73b40b8272267f
Signed-off-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37802
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Justin TerAvest <teravest@chromium.org>

Documentation/getting_started/index.md

@@ -7,3 +7,4 @@
 * [Gerrit Guidelines](gerrit_guidelines.md)
 * [Documentation License](license.md)
 * [Writing Documentation](writing_documentation.md)
+* [Setting up GPIOs](gpio.md)