ramips: add MT7615 wireless support for ELECOM WRC-1167GHBK2-S

ELECOM WRC-1167GHBK2-S has a MediaTek MT7615D chip for 2.4/5 GHz
wireless.

A driver package for MT7615 chip is added to OpenWrt in
a0e5ca4f,
so add preliminary MT7615 chip support for WRC-1167GHBK2-S.

Note: Currently, DBDC mode for MT7615 is not supported in mt76 driver.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>