dt: add documentation of "hotpluggable" memory property

dt: add documentation of "hotpluggable" memory property

Summarize the "hotpluggable" property of dt memory nodes.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Reza Arbab <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Bharata B Rao <[email protected]>
Cc: Frank Rowand <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Nathan Fontenot <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: Stewart Smith <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>