p4080ds: add README.p4080ds which documents the "serdes" hwconfig option

Add documentation for the "serdes" hwconfig option, which is used to
specify the status of SerDes banks two and three for the SERDES8 erratum
work-around.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>