config: enable more than 128 cores for arm64

config: enable more than 128 cores for arm64

When running dpdk applications on cores whose ids are bigger
than original max_core setting, eal error as below:
EAL: Detected 104 lcore(s)
EAL: Detected 2 NUMA nodes
EAL: invalid core list
-l CORELIST core indexes between 0 and 128

The fix is to increase max_core to 256 on arm64 platform.

Fixes: b3ce00e5 ("mk: introduce ARMv8 architecture")
Cc: stable@dpdk.org
Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>