nios2: Add CDX support

Add support for the CDX Code Density Extensions present in
Nios II R2 . This introduces new 16bit instruction set to
improve code density while retaining support for the 32bit
Nios II R2 instructions.

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