net: Fix client identifiers for ARM

There are client identifiers specifically reserved for ARM U-Boot
according to http://www.ietf.org/assignments/dhcpv6-parameters/dhcpv6-parameters.xml#processor-architecture

.

So let's actually make use of them rather than the bogus 0x100 that
we emitted so far.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Drop the Xilinx define to 0x100 as it's not the correct value to
use].
Signed-off-by: Tom Rini <trini@konsulko.com>