powerpc: Document sysfs DSCR interface

Add some documentation about ...

/sys/devices/system/cpu/dscr_default
/sys/devices/system/cpu/cpuN/dscr

... to Documentation/ABI/stable.
Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Documentation/ABI/stable/sysfs-devices-system-cpu

+What: 		/sys/devices/system/cpu/dscr_default
+Date:		13-May-2014
+KernelVersion:	v3.15.0
+Contact:
+Description:	Writes are equivalent to writing to
+		/sys/devices/system/cpu/cpuN/dscr on all CPUs.
+		Reads return the last written value or 0.
+		This value is not a global default: it is a way to set
+		all per-CPU defaults at the same time.
+Values:		64 bit unsigned integer (bit field)
+
+What: 		/sys/devices/system/cpu/cpu[0-9]+/dscr
+Date:		13-May-2014
+KernelVersion:	v3.15.0
+Contact:
+Description:	Default value for the Data Stream Control Register (DSCR) on
+		a CPU.
+		This default value is used when the kernel is executing and
+		for any process that has not set the DSCR itself.
+		If a process ever sets the DSCR (via direct access to the
+		SPR) that value will be persisted for that process and used
+		on any CPU where it executes (overriding the value described
+		here).
+		If set by a process it will be inherited by child processes.
+Values:		64 bit unsigned integer (bit field)