nios2: Add NIOS2_ARCH_REVISION to select between R1 and R2

Allow user to select between Nios2 R1 and R2. Since R1 and R2 are
not binary compatible, we cannot have a single kernel binary and
there is no point in having DT property for discerning these two.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Ley Foon Tan <lftan@altera.com>