ARM: sunxi_defconfig: enable NFS, TMPFS, PRINTK_TIME and nfsroot support

This enables a few more options on the sunxi defconfigs such that I can
use nfsroot to boot them (there is no local storage support yet). It
also enables PRINTK_TIME and tmpfs since it's a common distro requirement.

Signed-off-by: Olof Johansson <olof@lixom.net>