RFR: JDK-8051945 - Transfer DIO Spec From Internal Sources to Open Project

[Jen Dority]

issue: https://bugs.openjdk.java.net/browse/JDK-8051945
webrev: http://cr.openjdk.java.net/~jld/8051945/

Hi All,

Need a review for moving the javadoc sources for the DIO spec 
documentation into the Device I/O open project.

Note that this changeset adds stubs for jdk.dio.mmio package that is not 
yet supported for SE.

Thanks,
Jen