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>
diff --git a/configs/linkit-smart-7688_defconfig b/configs/linkit-smart-7688_defconfig
index 437000b..dd2524e 100644
--- a/configs/linkit-smart-7688_defconfig
+++ b/configs/linkit-smart-7688_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SPL_SYS_MALLOC_F_LEN=0x40000
 CONFIG_SPL=y
 CONFIG_ARCH_MTMIPS=y
+CONFIG_SOC_MT7628=y
 CONFIG_BOARD_LINKIT_SMART_7688=y
 CONFIG_DEFAULT_DEVICE_TREE="linkit-smart-7688"
 CONFIG_RESTORE_EXCEPTION_VECTOR_BASE=y