net: Remove unused mv88e61xx switch driver

No boards are using this driver.  Remove in preparation for a new
driver with integrated PHY support.

Signed-off-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>