sysfs: add attribute specification for /sysfs/devices/.../coredump

This patch adds the specification for /sysfs/devices/.../coredump
which allows user-space to trigger a device coredump obtaining
binary data from the device for (fault) analysis. It relies on
CONFIG_DEV_COREDUMP being enabled.
Signed-off-by: NArend van Spriel <aspriel@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-devices-coredump

+What:		/sys/devices/.../coredump
+Date:		December 2017
+Contact:	Arend van Spriel <aspriel@gmail.com>
+Description:
+		The /sys/devices/.../coredump attribute is only present when the
+		device is bound to a driver, which provides the .coredump()
+		callback. The attribute is write only. Anything written to this
+		file will trigger the .coredump() callback.
+
+		Available when CONFIG_DEV_COREDUMP is enabled.