char/bsr: add sysfs interface documentation

Descriptions have collected from code comments and by reading through
code.
Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/ABI/testing/sysfs-class-bsr

+What:		/sys/class/bsr/bsr*/bsr_size
+Date:		Jul, 2008
+KernelVersion:	2.6.27
+Contact:	Arnd Bergmann <arnd@arndb.de>,
+		Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+Description:
+		(RO) Size of the barrier-synchronization register (BSR)
+		register in bytes.
+
+What:		/sys/class/bsr/bsr*/bsr_length
+Date:		Jul, 2008
+KernelVersion:	2.6.27
+Contact:	Arnd Bergmann <arnd@arndb.de>,
+		Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+Description:
+		(RO) The length of memory region that can be mapped in bytes.
+
+What:		/sys/class/bsr/bsr*/bsr_stride
+Date:		Jul, 2008
+KernelVersion:	2.6.27
+Contact:	Arnd Bergmann <arnd@arndb.de>,
+		Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+Description:
+		(RO) The stride or the interval at which the allocated BSR bytes
+		repeat within the mapping.