ARC: [plat-*] ARC_HAS_COH_CACHES no longer relevant

A typical SMP system expects cache coherency. Initial NPS platform
support was slated to be SMP w/o cache coherency.

However it seems the platform now selects that option, so there is no
point in keeping it around.

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