target/riscv: Activate decodetree and implemnt LUI & AUIPC

target/riscv: Activate decodetree and implemnt LUI & AUIPC

for now only LUI & AUIPC are decoded and translated. If decodetree fails, we
fall back to the old decoder.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Peer Adelt <peer.adelt@hni.uni-paderborn.de>