configs: mtmips: refresh for mt7628 based boards

Since mt7620 is added into Kconfig, the CONFIG_SOC_MT7628=y which is
omitted by default must be added back, otherwise make xxx_defconfig for
these boards will be configured for mt7620 platform.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>