net: yaip: Moved ARP helper macro to arp.h

The helper macro NET_ARP_BUF() needs to be used from other
files so arp.h is proper place for it.

Change-Id: Ief7bbb385d9c180213ecd7cb08023f13999f5270
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>