Documentation: add sysfs entries for mtd docg3 chips

Add documentation for MSystems disk-on-chip docg3 chips
sysfs entries, which enable and disable protection areas,
giving or disabling access to the chip's memory.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

Documentation/ABI/testing/sysfs-devices-platform-docg3

+What:		/sys/devices/platform/docg3/f[0-3]_dps[01]_is_keylocked
+Date:		November 2011
+KernelVersion:	3.3
+Contact:	Robert Jarzmik <robert.jarzmik@free.fr>
+Description:
+		Show whether the floor (0 to 4), protection area (0 or 1) is
+		keylocked. Each docg3 chip (or floor) has 2 protection areas,
+		which can cover any part of it, block aligned, called DPS.
+		The protection has information embedded whether it blocks reads,
+		writes or both.
+		The result is:
+		0 -> the DPS is not keylocked
+		1 -> the DPS is keylocked
+Users:		None identified so far.
+
+What:		/sys/devices/platform/docg3/f[0-3]_dps[01]_protection_key
+Date:		November 2011
+KernelVersion:	3.3
+Contact:	Robert Jarzmik <robert.jarzmik@free.fr>
+Description:
+		Enter the protection key for the floor (0 to 4), protection area
+		(0 or 1). Each docg3 chip (or floor) has 2 protection areas,
+		which can cover any part of it, block aligned, called DPS.
+		The protection has information embedded whether it blocks reads,
+		writes or both.
+		The protection key is a string of 8 bytes (value 0-255).
+		Entering the correct value toggle the lock, and can be observed
+		through f[0-3]_dps[01]_is_keylocked.
+		Possible values are:
+			- 8 bytes
+		Typical values are:
+			- "00000000"
+			- "12345678"
+Users:		None identified so far.