arm: kirkwood: NSA310S: Add DM USB, DM Ethernet, and DM SATA configs

Convert to Driver Model.

- Add DM USB, DM Ethernet, and DM SATA configs to nsa310s_defconfig
- Add CONFIG_DEFAULT_DEVICE_TREE to nsa310s_defconfig
- Move CONFIG_ENV_SECT_SIZE from board file to nsa310s_defconfig
- Add CONFIG_IDENT_STRING, and CONFIG_NET_RANDOM_ETHADDR
to nsa310s_defconfig

Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>