lantiq: define SOC only once for uniform targets

lantiq: define SOC only once for uniform targets

In lantiq there are several subtarget where all devices have the
same value set to the SOC variable for each device individually.

This patch introduces a non-device-dependent variable DEFAULT_SOC,
which is used if no specific SOC is set for a device, and thus reduces
the number of redundant definitions drastically.

This is applied to all subtargets except xway, as only the latter has
two different SOCs.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>