FogBugz #212185: nios2: define "PG_arch_1" as PG_dcache_clean

Define PG_arch_1 as PG_dcache_clean and set it when a page is flushed.
Follow commit c01778001a4f5ad9c62d882776235f3f31922fdd for ARM.

Signed-off-by: Yuriy Kozlov <ykozlov@ptcusa.com>
Signed-off-by: Ley Foon Tan <lftan@altera.com>
FogBugz #212187: nios2: Avoid flush_dcache_page() for zero page

The zero page is read-only, and has its cache state cleared during
boot.  No further maintenance for this page is required.

Follow commit 421fe93cc4b06b2f5e875cbe0f692800d4862ee5 for ARM.

Signed-off-by: Yuriy Kozlov <ykozlov@ptcusa.com>
Signed-off-by: Ley Foon Tan <lftan@altera.com>
FogBugz #212188: nios2: Flush page regardless of if there is a mapping

This is how ARM does it.

This fixes a problem with applications such as libpcap reading stale
data from mmapped buffers.

Signed-off-by: Yuriy Kozlov <ykozlov@ptcusa.com>
Signed-off-by: Ley Foon Tan <lftan@altera.com>
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>