ARC: guard dsp early init against non ARCv2

As of today we guard early DSP init code with
ARC_AUX_DSP_BUILD (0x7A) BCR check to verify that we have
CPU with DSP configured. However that's not enough as in
ARCv1 CPU the same BCR (0x7A) is used for checking MUL/MAC
instructions presence.

So, let's guard DSP early init against non ARCv2.

Fixes: 4827d0cf ("ARC: handle DSP presence in HW")
Reported-by: Angelo Ribeiro <angelor@synopsys.com>
Suggested-by: Jose Abreu <oabreu@synopsys.com>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>