drivers: ethernet: Add TI Stellaris ethernet controller driver.

The driver can be tested using different networking emulation
approaches.

This approach will work across multiple Qemu instances. There can be
more than one Qemu instance, run using the following command. They
would appear to be on the same Ethernet network.

  $ qemu-system-arm -M lm3s6965evb                      \
                    -serial stdio                       \
                    -net nic                            \
                    -net socket,mcast=230.0.0.1:1234    \
                    -kernel zephyr.elf

This approach will work with other virtualization technologies that
support connecting to a VDE switch, like VirtualBox and User Mode
Linux. The switch can be started using the following command.

  $ vde_switch --sock /tmp/switch

Qemu can be connected to the switch using the following command.

  $ qemu-system-arm -M lm3s6965evb                      \
                    -serial stdio                       \
                    -net nic                            \
                    -net vde,sock=/tmp/switch   	\
                    -kernel zephyr.elf

Signed-off-by: Fadhel Habeeb <fadhel@zilogic.com>
Signed-off-by: Nirav Parmar <niravparmar@zilogic.com>
Signed-off-by: Vijay Kumar B <vijaykumar@zilogic.com>