ARM: owl: Drop custom machine

Rely on the fallback to "Generic DT based system".
This change is visible in /proc/cpuinfo.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>