ARM: dts: marzen: Add rw to bootargs and use ip=dhcp

Add rw as bootargs parameter and change from ip=on to ip=dhcp to make the
Marzen board bootargs match other boards from Renesas. No need to be special.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>