ARC: [dts] add cpu nodes to ARCHS SMP device tree

Trying to get clock for CPU cores on SMP systems I found that I was only
able to get clock for core[0]. That was because only one cpu@0 node was
represented in ARC HS device tree and it was impossible to get clock for
"non-existing" cores.

So as ARC HS may have up to 4 cores we update device tree to match
maximum possible cores quantity.

Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>