net: make ARP timeout configurable

Currently the timeout waiting for an ARP reply is hard set to 5 seconds.
On i.MX31ADS due to a hardware "strangeness" up to four first IP packets
to the boards get lost, which typically are ARP replies. By configuring
the timeout to a lower value we significantly improve the first network
transfer time on this board. The timeout is specified in milliseconds,
later internally it is converted to deciseconds, because it has to be
converted to hardware ticks, and CFG_HZ ranges from 900 to 27000000 on
different boards.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>