Prepare for v0.2-pre3 release

Update revision history block

Signed-off-by: Grant Likely <grant.likely@arm.com>

Author: glikely

source/revhistory.rst

@@ -8,7 +8,7 @@
    0.1       2016-MAY-24 Initial prerelease version. Imported |epapr| text
                          into reStructured Text format and removed Power ISA
                          specific elements.
-   0.2-pre2  2017-DEC-07 Second prerelease version.
+   0.2-pre3  2017-DEC-12 Third prerelease version.
 
                          * Added more recommended generic node names
                          * Added interrupts-extended