ramips: define SOC only once for uniform targets

ramips: define SOC only once for uniform targets

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>