FogBugz #214042: nios2: add nios2 gen2 dts support

Add nios2 gen2 dts support. Software will search for altr first and
then search for ALTR if can't find with altr prefix.
This is to backward compatible with nios2 legacy (with ALTR prefix).

Signed-off-by: Ley Foon Tan <lftan@altera.com>