nios2: Add architecture emulation support

nios2: Add architecture emulation support

Add support for emulating Altera NiosII R1 architecture into qemu.
This patch is based on previous work by Chris Wulff from 2012 and
updated to latest mainline QEMU.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chris Wulff <crwulff@gmail.com>
Cc: Jeff Da Silva <jdasilva@altera.com>
Cc: Ley Foon Tan <lftan@altera.com>
Cc: Sandra Loosemore <sandra@codesourcery.com>
Cc: Yves Vandervennet <yvanderv@altera.com>
Cc: Alexander Graf <agraf@suse.de>
Message-Id: <20170118220146.489-3-marex@denx.de>
[rth: Remove tlb_flush from nios2_cpu_reset.]
Signed-off-by: Richard Henderson <rth@twiddle.net>