sysinfo: Add gpio-sysinfo driver

This uses the newly-added dm_gpio_get_values_as_int_base3 function to
implement a sysinfo device. The revision map is stored in the device tree.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/device-tree-bindings/sysinfo/gpio-sysinfo.txt

@@ -0,0 +1,37 @@
+GPIO-based Sysinfo device
+
+This binding describes several GPIOs which specify a board revision. Each GPIO
+forms a digit in a ternary revision number. This revision is then mapped to a
+name using the revisions and names properties.
+
+Each GPIO may be floating, pulled-up, or pulled-down, mapping to digits 2, 1,
+and 0, respectively. The first GPIO forms the least-significant digit of the
+revision. For example, consider the property
+
+	gpios = <&gpio 0>, <&gpio 1>, <&gpio 2>;
+
+If GPIO 0 is pulled-up, GPIO 1 is pulled-down, and GPIO 2 is floating, then the
+revision would be
+
+	0t201 = 2*9 + 0*3 + 1*3 = 19
+
+If instead GPIO 0 is floating, GPIO 1 is pulled-up, and GPIO 2 is pulled-down,
+then the revision would be
+
+	0t012 = 0*9 + 1*3 + 2*1 = 5
+
+Required properties:
+- compatible: should be "gpio-sysinfo".
+- gpios: should be a list of gpios forming the revision number,
+  least-significant-digit first
+- revisions: a list of known revisions; any revisions not present will have the
+  name "unknown"
+- names: the name of each revision in revisions
+
+Example:
+sysinfo {
+	compatible = "gpio-sysinfo";
+	gpios = <&gpio_a 15>, <&gpio_a 16>, <&gpio_a 17>;
+	revisions = <19>, <5>;
+	names = "rev_a", "foo";
+};