ramips: provide common definition for ELECOM WRC GS devices

ramips: provide common definition for ELECOM WRC GS devices

Add a common definition for ELECOM WRC "GS" devices to mt7621.mk
to not repeat the same assignments five times.

To keep the naming consistent, slightly rename the DTSI and the
factory image recipe as well.

Note that elecom_wrc-1167ghbk2-s uses a slightly different build
recipe for the factory image, so we keep it separate.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: INAGAKI Hiroshi <musashino.open@gmail.com> [WRC-1750GSV]