arc: Don't halt slaves

arc: Don't halt slaves

This commit basically reverts two commits:
 1. dc654b110876 ("arc: arcv1: Disable master/slave check")
 2. 99a295f646a0 ("arcv2: Halt non-master cores")

With mentioned commits in-place we experience more trouble than
benefits. In case of SMP Linux kernel this is really required as
we have all the cores running from the very beginning and then we
need to allow master core to do some preparatory work while slaves
are not getting in the way.

In case of U-Boot we:
 a) Don't really run more than 1 core in parallel
 b) We may use whatever core for that

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>