am335x: Move platform headers to libethdrivers

They were previously located in libsel4platsupport which is the
incorrecct location. As they are specific to the ethernet hardware,
moving them here is better.

libethdrivers: fix licence headers for am335x files

SELFOUR-711: Ethernet driver for am335x (BeagleBoneBlack)

The driver is copied from StarterWare 02.00.01.01 Release, and modified
to work as a seL4 user-space driver. Some of the driver/board definitions from
libsel4platsupport are used to avoid duplications.

* new file: src/plat/am335x/beaglebone.c. This file is almost the same
as imx6.c with a very few additions from StarterWare driver. ring
buffers are re-implemented in imx6-like way unlike the original
StarterWare driver.
* new file: include/ethdrivers/am335x.h (Copied from StarterWare)
* new file: src/plat/am335x/cpsw/cpswif.c (Copied and modified from
StartewWare). Notable changes are removal of rx, tx handlers and ring
initialisation, and make it lwip-transparent. lwip handling are left to
lwip.c/lwip.
* new file: src/plat/am335x/cpsw/cpswif.h (Copied from StarterWare)
* new file: src/plat/am335x/lwiplib.h (Copied from StarterWare)

SELFOUR-711: Original unmodified copied files from StarterWare 02.00.01.01 release