w1: introduce an ability to specify microseconds bus scanning intervals

Some of 1-Wire devices commonly associated with physical access control
systems are attached/generate presence for as short as 100 ms - hence
the tens-to-hundreds milliseconds scan intervals are required.

Signed-off-by: Dmitry Khromov <dk@icelogic.net>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/stable/sysfs-bus-w1

@@ -0,0 +1,11 @@
+What:		/sys/bus/w1/devices/.../w1_master_timeout_us
+Date:		April 2015
+Contact:	Dmitry Khromov <dk@icelogic.net>
+Description:	Bus scanning interval, microseconds component.
+		Some of 1-Wire devices commonly associated with physical access
+		control systems are attached/generate presence for as short as
+		100 ms - hence the tens-to-hundreds milliseconds scan intervals
+		are required.
+		see Documentation/w1/w1.generic for detailed information.
+Users:		any user space application which wants to know bus scanning
+		interval