nios2: Add new architecture

This commit adds very basic support to build for the Nios II
architecture. Toolchain support is still missing and instead
we need to use an external custom toolchain.

Notice that this architecture had been previously removed in
Buildroot 2010.05-rc1 release (as explained in the CHANGES file)
and this commit adds it back.

Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>