CPU: Add NIOS2 support

The NIOS2 CPU is little-endian.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Ley Foon Tan <lftan@altera.com>
Cc: Thomas Chou <thomas@wytron.com.tw>
Cc: Walter Goossens <waltergoossens@home.nl>
Change-Id: Ifd1bc890b2fb74168b934641c70800a029f7f0b8