rtc: Add support for Intersil ISL12057 I2C RTC chip

Intersil ISL12057 is an I2C RTC chip also supporting two alarms. This
patch only adds support for basic RTC functionalities (i.e. getting
and setting time). Tests have been performed on NETGEAR ReadyNAS 102
w/ startup/shutdown scripts, hwclock, ntpdate and openntpd.

Reviewed-by: Mark Brown <broonie@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>
diff --git a/Documentation/devicetree/bindings/vendor-prefixes.txt b/Documentation/devicetree/bindings/vendor-prefixes.txt
index ce95ed1..507761a 100644
--- a/Documentation/devicetree/bindings/vendor-prefixes.txt
+++ b/Documentation/devicetree/bindings/vendor-prefixes.txt
@@ -38,6 +38,7 @@
 idt	Integrated Device Technologies, Inc.
 img	Imagination Technologies Ltd.
 intercontrol	Inter Control Group
+isl	Intersil
 linux	Linux-specific binding
 lsi	LSI Corp. (LSI Logic)
 marvell	Marvell Technology Group Ltd.