at91: enable USB ethernet for taskit stamp9g20

enabling CONFIG_MACB makes other locations in the stamp config file
enable network related commands (actually prevents disabling them)

enable USB ethernet support by activating generic support as well as
Asix and Moschip ethernet adapters

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Acked-by: Andreas Bießman <andreas.devel@googlemail.com>