doc/SPL/README.am335x-network: Document using ethernet (and USB) SPL

Added README file with the description of required options and host
configuration to use network SPL with am335x targets.  Briefly discuss
how to use this configuration to program empty boards.

Signed-off-by: Ilya Yanok <ilya.yanok@cogentembedded.com>
Signed-off-by: Tom Rini <trini@ti.com>