km/arm: remove unmaintained target PORTL2

Remove the PORTL2 board and all its dependencies as it is no longer
supported by the company.

Signed-off-by: Pascal Linder <pascal.linder@edu.hefr.ch>
Signed-off-by: Holger Brunck <holger.brunck@ch.abb.com>
Signed-off-by: Stefan Roese <sr@denx.de>