In ramips, all devices in mt7621, mt76x8 and rt288x subtarget 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.
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>