documentation: dt-bindings: mem-offline: Add doc for offline-sizes

Add documentation for offline-sizes which helps specify
an array of offlinable memory region sizes to apply to
targets based on their DDR sizes.

Change-Id: I9c356528799c969fecb65f287cc895fb4627dbbc
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>