rtc: cmos: Add OF bindings

This allows to load the OF driver based informations from the device
tree. Systems without BIOS may need to perform some initialization.
PowerPC creates a PNP device from the OF information and performs this
kind of initialization in their private PCI quirk. This looks more
generic.

This patch also avoids registering the platform RTC driver on X86 if
we have a device tree blob. Otherwise we would setup the device based
on the hardcoded information in arch/x86 rather than the device tree
based one.

[ tglx: Changed "int of_have_populated_dt()" to bool as recommended by
        Grant ]

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Cc: sodaville@linutronix.de
Cc: devicetree-discuss@lists.ozlabs.org
Cc: rtc-linux@googlegroups.com
Cc: Alessandro Zummo <a.zummo@towertech.it>
LKML-Reference: <1298405266-1624-12-git-send-email-bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/devicetree/bindings/rtc/rtc-cmos.txt

@@ -0,0 +1,28 @@
+ Motorola mc146818 compatible RTC
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Required properties:
+  - compatible : "motorola,mc146818"
+  - reg : should contain registers location and length.
+
+Optional properties:
+  - interrupts : should contain interrupt.
+  - interrupt-parent : interrupt source phandle.
+  - ctrl-reg : Contains the initial value of the control register also
+    called "Register B".
+  - freq-reg : Contains the initial value of the frequency register also
+    called "Regsiter A".
+
+"Register A" and "B" are usually initialized by the firmware (BIOS for
+instance). If this is not done, it can be performed by the driver.
+
+ISA Example:
+
+	rtc@70 {
+	         compatible = "motorola,mc146818";
+	         interrupts = <8 3>;
+	         interrupt-parent = <&ioapic1>;
+	         ctrl-reg = <2>;
+	         freq-reg = <0x26>;
+	         reg = <1 0x70 2>;
+	 };