realview: set LAN interface to be in DHCP

Realview is mostly used as a QEMU development platform, which will provide a
built-in DHCP server to NAT or bridge the connection outside of the emulated
environment.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41495 3c298f89-4303-0410-b956-a3cf2f4a3e73