drivers: counter: add Maxim DS3231 support

The DS3231 is an I2C real-time clock with internal temperature
compensated oscillator, maintaining civil time to 1 s precision with
nominal 2 ppm accuracy from 0-40 Cel.

The basic functionality is exposed as a counter that is always running
at 1 Hz.  Much more functionality is exposed as driver-specific API,
including the ability to translate between the time scale of the DS3231
and the time scale of the Zephyr uptime clock.  This allows correlation
of events in the system clock to UTC, TAI, or whatever time scale is
used to maintain the DS3231.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>

dts/bindings/rtc/maxim,ds3231.yaml

@@ -0,0 +1,39 @@
+#
+# Copyright (c) 2019 Peter Bigot Consulting, LLC
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+description: Maxim DS3231 I2C RTC/TCXO
+
+compatible: "maxim,ds3231"
+
+include: i2c-device.yaml
+
+properties:
+  reg:
+    required: true
+
+  32k-gpios:
+    type: phandle-array
+    required: false
+    description: |
+
+      32 KiHz open drain output
+
+      The DS3231 defaults to providing a 32 KiHz square wave on this
+      signal.  The driver does not make use of this, but applications
+      may want access.
+
+  isw-gpios:
+    type: phandle-array
+    required: false
+    description: |
+
+      interrupt/square wave open drain output
+
+      The DS3231 uses this signal to notify when an alarm has triggered,
+      and also to produce a square wave aligned to the countdown chain.
+      Both capabilities are used within the driver.  This signal must be
+      present to support time set and read operations that preserve
+      sub-second accuracy.