Documentation/ABI: Document umwait control sysfs interfaces

Since two new sysfs interface files are created for umwait control, add
an ABI document entry for the files:

   /sys/devices/system/cpu/umwait_control/enable_c02
   /sys/devices/system/cpu/umwait_control/max_time

[ tglx: Made the write value instructions readable ]
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Cc: "Borislav Petkov" <bp@alien8.de>
Cc: "H Peter Anvin" <hpa@zytor.com>
Cc: "Andy Lutomirski" <luto@kernel.org>
Cc: "Peter Zijlstra" <peterz@infradead.org>
Cc: "Tony Luck" <tony.luck@intel.com>
Cc: "Ravi V Shankar" <ravi.v.shankar@intel.com>
Link: https://lkml.kernel.org/r/1560994438-235698-6-git-send-email-fenghua.yu@intel.com

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

@@ -538,3 +538,26 @@ Description:	Intel Energy and Performance Bias Hint (EPB)
 
 		This attribute is present for all online CPUs supporting the
 		Intel EPB feature.
+
+What:		/sys/devices/system/cpu/umwait_control
+		/sys/devices/system/cpu/umwait_control/enable_c02
+		/sys/devices/system/cpu/umwait_control/max_time
+Date:		May 2019
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:	Umwait control
+
+		enable_c02: Read/write interface to control umwait C0.2 state
+			Read returns C0.2 state status:
+				0: C0.2 is disabled
+				1: C0.2 is enabled
+
+			Write 'y' or '1'  or 'on' to enable C0.2 state.
+			Write 'n' or '0'  or 'off' to disable C0.2 state.
+
+			The interface is case insensitive.
+
+		max_time: Read/write interface to control umwait maximum time
+			  in TSC-quanta that the CPU can reside in either C0.1
+			  or C0.2 state. The time is an unsigned 32-bit number.
+			  Note that a value of zero means there is no limit.
+			  Low order two bits must be zero.