ARC: [plat-eznps]: Restrict to CONFIG_ISA_ARCOMPACT

Elide invalid configuration EZNPS + ARCv2, triggered by a
make allyesconfig build.

Granted the root cause is in source code (asm/barrier.h) where we check
for ARCv2 before PLAT_EZNPS, but it is better to avoid such combinations
at onset rather then baking subtle nuances into code.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>